There are many reasons the reported sizes may be different. It depends on what they are actually measuring as "blockchain size".

A naive way of measuring would be to just take the size of the datadir for a bitcoind instance. However, this is going to include a bunch of extra data which is not actually the blockchain. This would include data like log files, wallet files, the UTXO set database, the block index, and the transaction index if that is enabled.

There may also be a discrepancy if one site does not run a segwit enabled node. Since a Segwit block is physically larger than the stripped block that a non-segwit node would receive, sites that support Segwit will report a larger blockchain.

Additionally, a site may receive more orphan blocks than another site which means that they are storing more blocks on disk. They may be measuring this as well which will cause the reported blockchain size to differ from sites that have received different orphan blocks.