ISO and JIS are both a 2deg taper, but an ISO is marginally smaller at the end than JIS.
The end result is that if you put an ISO crank on a JIS BB it doesn't go on as far as it should, and a JIS crank on an ISO BB goes on too far. The difference is about 3 or 4mm or so on what the chainline should be.
Problems mostly occur with JIS crank on ISO BB, where going on too far means the crank bolt gets tight on the end of the axle before the crank itself is tight on the taper. Tolerances are such that it's sometimes OK, and sometimes it isn't.
( I hope I've got the difference the right way round
)