A few things I can think of:
Oxygen is distributed by the centre (in theory) on the basis of what states need.
In practice there are imbalances so who knows.
If you look at the efficacy of lockdowns (which are being imposed by states rather than the centre now), there is no way of knowing whether they work or not if some states do not report.
Many (most? all?) states require a positive test for hospital admission. If tests are being fudged...
Political point scoring - 3 of the 4 states with lots of reports of undercounting are ruled by the party that also rules at the centre. In theory it can be a persuasive argument to win elections.
Anyway, my post was in response to someone claiming that our per-capita numbers aren't bad compared to the UK, and my point was that our per-capita numbers don't mean much.