'Til is a MADE UP WORD!
Till is the short version of until. Till actually came before until - un was added later.
It takes considerably longer to write 'til than till - till is virtually 3 characters due to the double L, whereas 'til is 4 characters including a fiddly apostrophe. Plus, 'til is a fake word.
Dumb Yanks.
Before you go off on a "dumb Yanks" rant, keep in mind that
* both << 'til >> and << till >> are four characters (two "l"s are two characters, not "virtually" one);
* there are legitimate distinctions between AmE and BrE (such as the absence or use of "u" in words such as "colour" and "favourite");
* an apostrophe designates omitted letters, as in contractions (ex.: "doesn't", where the apostrophe stands in for the omitted "o" of "not"), meaning that "until" -- which has one "l" -- is more legitimately shorted to << 'til >> than to << till >>; and
* << 'til >>, though archaic and self-conscious, was probably used in poetry in an effort to maintain rhythm, as were << 'neath >> and "o'er".
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