i don't know about other but my prediction is based on games that look to me similar to Bloodborne like other Soul games,Heavy rain,Beyond: Two Souls none of them sold more then 750k first week like i said before The last of us is the only Sony new IP that was able to sell more then 750k first week in 7 and 8 gen.
It's not like all people predict low sale for Bloodborne some predict it do do very well some predict it will do ok some think it will flop just like any other games.