Their are a lot of reasons that perceived velocity varies among guys that actually throw at very similar speeds. In addition to those already mentioned I would throw in my 2 cents. All reasons have to do with illusion. There is something that particular pitcher does that appears different to the eye in some way. One mechanical way is when a pitcher keeps his shoulders in line with the target when initially separation of the hands, and has a short arm circle that is also in line with the target, the batter can’t track the ball during the process of the pitcher delivering the pitch. He doesn’t see the ball until it’s out of the pitcher’s hand. This creates the visual image of the ball getting on the hitter quicker than expected and the hitter’s mind tells him that it’s a higher than actual velo. A high spin rate FB that holds it’s plane can have the same effect, although not so much until you get velos 80s and above. Funky deliveries that distract the eye of the batter can have the same effect, although I’m not a fan of players that do that intentionally. I’m good with it if it’s organic.