there are several thoughts come to mind....It is very possible that the previous prescriptions were what we call over minused. What that simply means is that during the exam, your eyes were not fully relaxed and your eye basically liked to take up more minus power during the exam. You see, you can actually take more minus power than needed, your lens in the eye (behind your pupil) simply accomodates more than it has to bring things into perfect focus. A young person can tolerate this well, older folks will get headaches and eye fatigue. So if your current prescription seems more comfortable, this is possible.
Second, is that astigmatism has slightly improved and that leads to an overall reduction in the amount of minus in the lenses (this means less negative power).
Third is that your corneas are becoming more flat as you age, this is possible and leads to an overall reduction in minus power needed.
Fourth, and this is unlikely, is that there is something that is causing the eyes to become shorter in lenght such as some increase in orbital fat or tissue, this can be seen in things like hyperthyroid graves disease, rarely tumors. Typically the scenario is that a person was -5 in each eye and comes back a year later and they are -2 and -5....the point is there may be a tumor in the -2 eye socket. These are all the things that could be playing in this, my guess is a combo of scenario 1,2 and 2 above. It is even possible that you are more likely around -3 or -3.5 and can still see relatively well in the opposite direction. The reason is that contacts can bend your cornea and induce a bit more power in the cornea. Lots of optics issues come into play, I hope this helps, Dr. Stevens