First, a phoropter (that machine full of lenses that is swung in front of your face) has quite a bit of error range, especially if you went to two different ophthalmologists; and you didn’t mention how long it has been between measurements (age difference?). Secondly, your vision can vary from day to day, depending on your diet and other factors. So my interim answer is that this much variation is not necessarily a slippery slope toward weaker vision in toto. But to address your question directly: there are eye exercises, but they mainly affect your ability to change focus lengths. Any inordinate screen use (say, more than eight hours a day) can limit your farsighted ability – the preventive activity is to look at a distance at least every two hours or so – take a scenic drive, gaze at the distant mountains, etc. Your optometrist should be able to give you more advice. Also, ask youself if you notice any difficulty reading small print.