I think you had a combination of a close match and a worn ignition on the older car. I can't speak for Honda, but some manufacturers may have more tumblers in the door locks than in the ignition locks, probably more to make a more compact lock for the steering column than anything else. I know glove compartment locks are often this way. Eg. a door lock may have 5 discs while an ignition lock has 4. This means that you have a lot more chances for a key from another car to start yours, while it may not open your doors, due to one extra matching key cut being necessary for the extra tumbler in the door lock.
I have actually seen this on an old Russwin double-cylinder (house) deadbolt, where the inside cylinder was built into the horseshoe-shaped deadbolt unit, and only had 3 pins, using the 3 cuts closest to the key's head, while the outside cylinder had 5 pins. The unused length of the key stuck out the back of the cylinder inside the unit. Some master key systems that contain a large number of locks with several levels of master keying and desired access groups (like for a large facility or campus for example) may also use this system, with some locks having all pins, some having one less, and some missing a pin in different positions in the cylinder to accommodate the convoluted key schedule, though these type of systems typically have at least 6 pins for greater security, and may have the entire system duplicated at least once, but with more than one keyway--so that two keys can have identical cuts, but would not fit into cylinders meant for each other.