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 uint256s.

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 with uint8s, because most of the writes overwrite existing storage to cram more stuff in an existing slot, rather than expanding storage.

When you delete, absent optimizations, you have the same number of writes either way, 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 uint256s.