A "Barking" rhythm trivia question

If the drivers of a locomotive are exactly quartered between the two sides, I'd think that the rhythm of four stack exhausts per wheel revolution would be evenly spaced and of the same intensity. And I find that's usually what's heard. But with some engines I've heard, four exhausts are followed by a slightly longer period before the next sequence of four, and/or sometimes the first of the four "barks" is louder than the following three - - i.e. "CHUG, chug, chug, chug, .......CHUG, chug, chug, chug," etc. I'd be very interested in any comments on the reason(s) for such variations, if anyone thinks it's sufficiently worthy of your time. Thanks!