It's an extremely minute amount of time, but they have to be calibrated. It's due to the speed they are traveling. Einstein, sure. But they look to be in the same time as us seen through a telescope. I just can't wrap my brain around it.
You're right, it is due to the speed they are traveling. Velocity distorts time by the factor: t'=t/sqrt(1-v^2/c^2) where t' is the time that passes for a stationary observer, t is the time that passes for the object moving at the high velocities, c is the speed of light, and v is the velocity the object is moving.
This effect is basically negligible unless the object is moving close to the speed of light, but the orbiting satellite is moving pretty fast for a very long time, and so even though it's traveling at a very small velocity relative to the speed of light, it still builds up a little and creates a small distortion of time that is barely noticeable. For example, let's say the satellite is traveling at 8,000 m/s for 5 years. if you do the math, there are 157680000 seconds in 5 years. The speed of light is 299792458m/s. Now lets see how much this effects time. That number will be t', since it's how much time passes on earth for 5 years. Now let's see about on the satellite:
157680000=t/sqrt(1-(8000^2/299792458^2) solving for t, (must have fancy calculator with a LOT of places) we get t= 157679999.88772s subtracting this from the original time, the clock is behind by .11228 of a second.
Time moved slower for the satellite, but since the velocity is so small compared to the speed of light, the amount that is different is so insignificant, that you wouldn't be able to see the difference through a telescope. However, they want the clocks to be perfectly calibrated and exact, so they do that to make up for this small effect because it would build up more and more as time went on, and the faster the satellite is moving, the more time is distorted.