I’m struggling with “enroll” and “enrollment”. Both answers (this one and this one), given to this question, as well as Wikipedia seems to be suggesting, that double “l” is more common in British English, while single “l” in American English.
Is “enrollment” some kind of exception (and if yes, then what kind)? Or why double “l” is sometimes considered British English and other time — American English?
You have to keep in mind that <l> and <ll> are both extremely common in English, regardless of region. For example, bill is always spelled <bill>, and nil is always spelled <nil>; excel is always spelled <excel>, and retell is always spelled <retell>. There are a lot of individual rules, but there’s no single over-arching pattern, and the rules don’t cover everything. For example, how come lily has one L, while filly has two? (In this respect, <l> vs. <ll> is much like many other aspects of English spelling. English spelling is not quite as random as people like to claim, but, well, there is a lot of arbitrariness.)
So, what you’ve noticed is that there are (at least) two cases of regional variation involving the choice between <l> and <ll>:
- in enrol/enroll and enrolment/enrolment (and a few other such words, such as fulfil/fulfill and fulfilment/fulfillment), the <l> spelling is British and the <ll> spelling is American;
- in words like traveled/travelled and labeled/labelled, the <l> spelling is American and the <ll> spelling is British. (Though labelled and travelled and so on aren’t actually all that rare in the US — or they weren’t until spell-checkers became prevalent — so this distinction may be exaggerated.)
These may seem contradictory, but really it’s just that they’re basically unrelated. Both are cases where the arbitariness of <l> vs. <ll> has shaken out differently in the US as in the UK, but there isn’t much reason to expect that one region would have happened to end up with <l> in both cases and the other with <ll> in both cases, because there’s no connection between the two. Both regions still have <l> in thousands of words and <ll> in thousands of words — mostly the same ones — and these are simply two edge-cases where words got grouped differently on the two sides of the pond.