Each write operation costs gas. If it expands storage, it costs more gas. Completely clearing a storage slot refunds gas. The refund relates to storage slots, not array elements.

Storage slots are 32-byte, so a `uint256` fills a whole slot, whereas a `uint8` only fills part of a slot. This means that your adding case is cheaper, because most of the writes overwrite existing storage, rather than expanding storage.

When you delete, absent optimizations, you have the same number of writes, but in the `uint8` case most of them are updating the value in the slot, rather than clearing it. You only clear a slot when you remove the last `uint8` in it. So you get higher refunds when clearing the `uint256`s.