btc.com seems to use the tx fee from the last tx in the latest found block
there is no written explanation on the site, but I checked it manually and the number seems pretty close
and coinb.in does similarly but using from recent block (2 previous block) and use it for minimum Recommended Fee
latest block is 507015 and coinb.in uses tx in block 507013 for estimation
last txid e83c174722c6e766b45502251805a015b5e1cdfa521363fa130e0622c73c2247 in block 507013
This is based on us comparing your transaction against a very recent transaction found in a very recent block
Block Height: 507013
Block #: 0000000000000000003b19530456fbdff45dd9d698c79154bc865326cf9f0a1b
Block Time: 1517441134 31/01/2018 23:25:34
Transaction ID: e83c174722c6e766b45502251805a015b5e1cdfa521363fa130e0622c73c2247
Transaction Size: 216 bytes
Transaction Fee: 0.00021016
Satoshi per Byte: 97