I'm curious why they don't fully load it, I can speculate:
0) starship is still in testing, they don't want to change too many variables from flight 14 and add 50t of payload.
1) lower payload gives them more options in engine-out situations. On flight 14, both booster and ship had engines fail.
2) derisking for ship failure, each Starlink sat costs ~$1.2m, and losing $30m of satellites hits less hard than $60m if orbital insertion fails.
3) the dispenser installed on ship 42 only fits 26 sats, and increasing this would require a hardware modification and possible delays.
4) Starlink v3 manufacturing is delayed and they don't have enough units, which I find doubtful given Starship is years late and they should have a backlog of v3s waiting to launch.
5) block 3 has worse performance than expected and can't reliably deliver 100t to orbit. Recent comments that raptor 3 can only reliably deliver 230t of thrust instead of 280t support this. I speculate SpaceX is running raptor 3 >230t on booster ascent, which is why we keep seeing engines failing. This should be resolved by block 4 or 5 with raptor 4.