It's how it works in the trading world. The base is decently competitive (for devs anyway) but the bonus is based on trading results and functions similarly (from an incentives/alignment and retention perspective) for how stock works for other companies.

I think the point is that salary + bonus = your real salary, but they give much if it in the form of a bonus so they can more easily choose to give your overall "salary" a cut.

A trading shop can afford to give bigger bonuses to a smaller set of staff, not everyone who is still cutting teeth.