Servos, espeically inexpensive ones like the 425 do, in fact, vary. They use inexpenseive motors and pots in there, resulting in slight speed and travel distances. So yes, what you are seeing is normal. (note, this isn't specific to Hitec, all inexpensive servos are like this)

To fix it, you need to do one of several things:

Ditch the Y harness and use two channels and mixing. Then you adjust the mix %'s so that the travel is the same on both sides. You can't really fix the speed this way, but if they are close, you won't notice it in the air.

or

Get a pile of servos and go though them until you find a couple that are "matched"

or

Spend a lot more money on higher quality servos, Hitec or other brand, it's not the brand, it's the price-point. But even the best servos to vary a little bit.

or

Get some Hitec digitals and a digital servo programmer, this allows you to set each servo's endpoints, travel speed, and a couple other parameters to match servos up.

or

Get a "match box" or equilivent. This is a box that goes between the RX and the servos that allows you to adjust the speed, center, and end points (maybe more) of sevos to match them up. These things were designed for the guys who gang multiple servos on to one control surface to get higher torque than they could get from one servo alone. With out this kind of matching, the servos will fight each other.