The average is roughly
targetGas because eip-1559 is designed to adjust prices such that the average block has
targetGas. Most blocks do not contain exactly or close to
targetGas, that link is looking at averages per day. Scroll through https://etherscan.io/blocks a little to see the kind of variance I mean.
Builders generally include the top x valid transactions that also have a high enough minger tip to be worth the marginal cost of inclusion (I believe it's in the 1-2 gwei range), while remaining below
2 * targetGas gas. There is no incentive otherwise influencing the over/under
Yes going over
targetGas will increase base fee, which means the existing transactions in the mempool would net less priority fees in the next block, but that will only benefit the following block builder, not the current one. That's not accounting for the additional transactions that come through between this block and the next block, either. In reality the builder would likely be losing more by not including transactions than they would gain by artificially limiting base fee growth