In British English, words that end in -l preceded by a vowel usually double the -l when a suffix is added, while in American English the letter is not doubled.  The letter will double in the stress is on the second syllable.

The first category involves verbs that use -ed or -t for the simple past and past participle.   Generally, the rule is that if there is a verb form with -ed, American English will use it, and if there is a form with -t, British English uses it.  However, these forms do not exist for every verb and there is variation.  For example, both American and British English would use the word 'worked' for the past form of 'to work', and in American English it is common to hear the word 'knelt' as the past tense of 'to kneel'.

The second category of difference includes verbs that use either the base form of the verb or the -ed ending for the simple past.

The third category of difference includes verbs that have either an irregular spelling or the -ed ending for the simple past.

So what does tall his mean for learners of English?  In the beginning, unfortunately, it means a lot of memorization (or memorisation) and of course, a few mistakes.  For spoken English, the differences are barely audible, so forge ahead and don't be too concerned with whether a word is spelled 'dwelled' or 'dwelt'.  With written English, however, if you are unsure about the spelling, better to ask your teacher or look the word up in the dictionary and see what the experts say.