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.