I guess one way would be repeated on-off causing wear of the threads (especially in aluminum).
There has to be a little play between the external bolt threads and the internal nut or hole threads, otherwise it would be impossible to screw in the bolt. This is especially true if screwing a bolt into a deeply threaded hole, where there are a lot of threads interacting. That play is taken up when the bolt head tightens against the washer or whatever it tightens against.
Understanding all there is to know about bolts and nuts and studs would make for a full time job. When I got a little curiosity about this topic myself, I visited my local hardware store at a time they weren’t busy and just studied all the nuts and bolts and fasteners, washers, etc they had on display there, experimenting with what fits on what. And what different types of the same size fasters they have. I spent 2-3 hours there looking and experimenting, and came away with a much better understanding. I was hoping the staff there would be able to answer some questions, but no luck, all they knew about is the product numbers, skew numbers they call them. Once you go to into a hardware store these days, it seems you are on your own.
Automobiles stress fasteners more than in other uses, so sometimes specialty fasters are needed just for automobiles. There’s a company — ARP — that makes some of those, you might check what they have to say on their website.
No, it doesn’t. The hole (or nut) that the bolt has to go into has to be LARGER by an ever so slight amount - and that is what “Class of Fit” is all about.
So normally you can get a bit of a wiggle in a threaded faster before it is fully tightened.
Please note, there are some classes of fit where the bolt is LARGER than the hole and in order to install these, heat and/or cold is involved.
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That’s the engineer in you coming out! 
I Of course you are correct! I always value your comments and learn from you. I gave your comment a “like.”
I believe it’s a case of an engineering or technical point of view compared with novice DIYer nomenclature.
Technically speaking, the hole for an M8x1.25 bolts is larger than the bolt. However, from a simplified DIY “mechanic’s” viewpoint, they’re the same.
Let me explain…
I just had to remove the flywheel on my self-propelled walk-behind lawn mower. I had a harmonic balancer puller that would work, and there were 3 holes in the flywheel in the proper positions, BUT they weren’t threaded!
I used my 1/4-20 (1/4" x 20 threads per inch) tap and quickly cut threads into each hole. Then I found three 1/4-20 bolts of a suitable length and Voila! The flywheel came right off.
I realize the holes were technically larger than the bolts, but both the bolts and tap were, practically speaking, the same.
Does that make sense?
CSA
CSA,
Yup, I tend to be very specific - almost anal - about these sorts of things. All too often there is miscommunication. I’m even in the middle of one now on another website.
I love it!
CSA
Tell me if I’m wrong. The difference here, in my simple terms, is one between the “actual size” (exact measurement) and “nominal size” (what that size is called).
Both a bolt and nut could be described as having a nominal size of 1/4-20 (that’s what they’re marketed as), but we know that’s not necessarily the exact sizes.
CSA
CSA,
I think that may be the case here. We had 2 different conversations going on and I didn’t want anyone to get confused that the actual bolt dimensions were identical to the actual nut dimensions.
If people are interested in this topic, get this book: Carroll Smith’s Nuts, Bolts, Fasteners and Plumbing Handbook (Motorbooks Workshop)
Carroll Smith schooled a whole generation of amateur racers in the 70’s, 80’s and beyond on the ins and outs of race car preparation. His writing is funny and engaging and the guy just flat knows his stuff.
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I believe they are actually called SQU numbers, though I wouldn’t be surprised if some enterprising company decided to call them skew numbers since that’s how they’re pronounced.
Edit: see TwinTurbo’s response below, SKU not SQU. Now for more 
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SKU is how I’ve always heard it used- Stock Keeping Unit…
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Yes, that is correct. My brain apparently hadn’t fully loaded up this morning when I started typing…