Readings do tend to differ slightly, well up to 0.5 of a reading sometimes but if they are hugely different then you should be a little worried.
Peoples sight does change, and optitians do have to go through a process of trail and error as to which lense would suit you best and this can come up with different results.
Have you considered going to an independant optitian that has more time to sit down, and have a thorough assessment?