It is because these are two different "ranking" systems.
Levels (1, 2, ... 109 ...) are bound to the no. of experience points (XPs) you earn by diligence - playing strokes, finishing rounds of any kind, playing on consecutive days (bonus up to 880 XPs per day), level up with money...
Tiers (Hack, Amateur, ... Legend) are bound to your average in ranked strokeplay rounds and to a minimum of ranked strokeplay rounds per tier. This is meant to be a skill based ranking.
Now, s.o. with low tier and high level either
- lacks skills despite many rounds he played (high average) or
- deliberately omits to play enough ranked strokeplay rounds to be tiered up (typically low average).
The latter, also called sandbaggers, benefit from the good clubs they get with their high level while also benefitting from the forward tees they are allowed to play with their low tier. By this, they may be more successful, possibly win more matches and challenges.