That is very normal. It all has to do with focal lengths, someone who has had optics can describe it better. The Rx in the glasses sits further from your retina than the contact. Therefore, they need different focal lengths so the retina can focus of the image. My sister's glasses Rx is a OD-12.00 OS-12.50, her contacts a like a OD-9.75 OS-10.00. I could be off on this, I don't know how it all works. But what I'm trying to say is, your OD is right. A "lower" rx, like in the 3's is only going to have a quater diapter or so difference between them. While higher ones can have a whole diapter or more difference.
If you remember from physics, P=-1/f
f=focal length (m)
P=power (diapters)
The neg. sign indicates a concave lens
Your glasses sit about 2cm from your face, while your contacts are basically at a distance of 0cm. Your Rx of -3.75 means the image is going to be projected at a distance of 0.267 m. The Math: P=-1/0.267=-3.75. Now add the 2 cm to the distance so the image will be projected further away to find the contact lens Rx. P=-1/0.286=-3.50.
Now look at my sister:
Glasses=P=-1/.08=-12.50
Contacts=P=-1/.10=-10.00
Got to love Physics!!!
