The results of my distance-vision test this morning were a little better than normal. In fact, my weaker eye scored its best ever. (We only got the "denominator" into double digits in the last year...) I commented on this and my ophthamologist said it might be partially due to her newer equipment: the contrast is better on the new LCD "eye chart" than it was on the old projection chart, which in turn gives better readings than the posters of yore. (Personally, I think my gadget-assisted glasses prescription helps, particularly in the weaker eye.)
Measured visual acuity depends on the equipment. What it also depends on (based on my own observation) is operator variation. Your vision score includes a judgement call by the person administering the test. Whether you get an extra point can depend on how quickly or how certainly you read a letter. When you say "um, I think it's an F -- no, wait, it's a P", what happens to your score is not well-defined.
This doesn't really matter for an individual patient with a consistent doctor (presumably what the test was designed for); what matters is not so much your raw score but whether and how it changes from year to year. But when that score is used for other purposes, like deciding who can drive and who can fly a plane, it gives me pause. According to today's eye test, if both of my eyes were as bad as my weaker one I would still be allowed to drive (albeit only during daylight). Yikes.