Block validation rules do not strictly require that a block's timestamp be more recent than the timestamp of the block preceding it. Without a central authority, it's impossible to know what the exact correct time is. Instead, the Bitcoin protocol requires that a block's timestamp meet certain requirements. One of those requirements is that a block's timestamp cannot be older than the median timestamp of the 12 blocks that came before it. See more details here.
As a result, timestamps are only accurate to within an hour or so, which sometimes results in blocks with timestamps that appear out of order.
+Mempool aims to show you the effective feerate range for blocks—how much would you actually need to pay to get a transaction included in a block.
+A transaction's effective feerate is not always the same as the feerate explicitly set for it. For example, if you see a 1 s/vb transaction in a block with a displayed feerate range of 5 s/vb to 72 s/vb, chances are that 1 s/vb transaction had a high-feerate child transaction that boosted its effective feerate to 5 s/vb or higher (this is how CPFP fee-bumping works). In such a case, it would be misleading to use 1 s/vb as the lower bound of the block's feerate range because it actually required more than 1 s/vb to confirm that transaction in that block.
+For unconfirmed CPFP transactions, Mempool will show the effective feerate (along with descendent & ancestor transaction information) on the transaction page. For confirmed transactions, CPFP relationships are not stored, so this additional information is not shown.
+