(Warning, both math and word nerdiness follows)

Colloquial language is not statistically precise, so "likely" does actually mean about the same thing as probable. The catch is that generally we're thinking about binary outcomes. "BYU is likely to win tomorrow" In that case there are only two possibilities and likely is the same thing as probable.

Think about it this way, it's two very different statements to say that "BYU is likely to score at least 24 points tomorrow" and "BYU is likely to score 24 points tomorrow" That's why we use "most" likely to refer to something that has a higher probability than any other outcome, but may not itself be probable.

And showing how language is weird and ironic, saying something is "likely" is a stronger statement than saying something is "most likely".