You will have some variation in connectivity from pitch to pitch, which isn't uncommon in younger or less experienced pitchers. As you repeat your mechanics better, velocity will be more steady.
Also intent may be a significant factor.
I find myself sometimes unconsciously reducing my intent with a 3-ball count if I need to throw a strike. And that will always be my lowest velocity reading for the day (as low as 86-88 at times when I do that), and then when I am in attack mode as usual I will range as much as 7 or 8 mph faster (usually topping out between 93-95).
Again, if you have these massive variations from pitch to pitch, it is likely inconsistent mechanics, possibly as a result of an inconsistent mindset - focusing on your pre-pitch routine, including a deep breath, same focal point and a keyword to get you locked in, will help you repeat.