Sorry for the necro, stumbled across this thread while looking into the matter, and the last response is a bit vague, so...
Levels 20-16 are indeed directly linked to the ladder rank:
Rank | Level |
1-2 | 20 |
3-13 | 19 |
14-36 | 18 |
37-80 | 17 |
81-200 | 16 |
If you are not in the top 200, the level is determined based on where you are in the ladder relative to the other players. E.g. if your rank is better than the bottom 6% of the players for that faction/mode you get level 2. The full table is
Better than bottom | Level |
6% | 2 |
14% | 3 |
20% | 4 |
25% | 5 |
35% | 6 |
45% | 7 |
55% | 8 |
62% | 9 |
69% | 10 |
75% | 11 |
80% | 12 |
85% | 13 |
90% | 14 |
95% | 15 |
As a result, for lesser played factions/modes there are no players at levels 15 or 14, because being better than e.g. 90% of the UKF/1v1 players (level 14) already brings you in the top 200, where the rule for levels 16+ applies.
I'd argue that "level" is the best measure of success in the game in particular for cross faction comparisons; assuming that I'm about equally (un)skilled in what I play: Although my ranks are between 835 and 3980, my level is 8 almost everywhere, so this seems to work out well at least for me...