What Causes The Decrease In Eye Sight Within Two Months?

At the start of last year, my eyes were examined and they were RH eye: -3.25, LH: -2.75 wiith -0.5 astigmatism. (My previous prescription was -2.75 in both eyes with mild astigmatism)

Then, towards the end of last year around end of October, after my university exams, I stopped wearing my glasses because throughout the year, I constantly find slight discomfort from wearing them. I started looking at distant green things, and a *little* bit of eye exercies, and also, one thing I forgot to mention is that, the main reason I stopped wearing them was I developed double vision which worsens after wearing them.

After, about 2 weeks of not wearing my glasses, I decided to go and see the optometrist and have my vision checked. To my surprise, my new prescription decreased to RH:-2.25(with -0.25 astigmatism) and LH:-1.75 (-0.5 astigmatism). I decided to wait and not get glasses as I was going overseas to asia for a holiday, and I thought I might get glasses over there as they are less expensive. I stopped wearing glasses, and spend slightly more time outdoors playing sports as my exams were over.

Around early December, I had my eyes examined in my holidays in China, and once again, my prescription decreased to RH:-1.5, and LH:-1.25 with similiar astigmatism.I decided to buy two top quality glasses from Levis, which costed quite a lot, and I started wearing the glasses. On February, back from my holidays, I decided to go back to my regular optometrst to have my eyes examined to confirm my prescription, and to his surprise, the presciption on my glasses was indeed RH:-1.5, and LH:-1.25 (with mild astigmatism). I asked him why my eyesight improved, and he cannot quite explain why either, he suggested maybe there was some sort of strain in my eyes when we did the examinations, which caused a non accurate prescription.

I am currently a tertiary student, and I got glasses ever since I was 13. I think my first prescription was around -1.75 with no astigmatism, and about one year later, it changed to -2.75, which was stable till start of last year.

Can someone please help explain this eyesight change (my presciption decreased in half over just two months)?