It's probably down to a combination of catching a glimpse of yourself unawares, perhaps when you're unintentionally pulling a funny face, which makes you look weird, and also the unflattering bright lights (often strip lights) in shops that tend to 'bleach out' skin tone and distort your image - all of which is quite different from at home, when you are *deliberately* looking at yourself in the mirror and controlling what you see under softer lighting.
It's a bit like when someone takes a picture of you unawares, compared with the results obtained when you're *posing* for one. Rarely, in my experience, does the former look good, or complimentary - and it's similar when catching your reflection in a mirror unexpectedly.
I also think you tend to look more natural the closer you are to a mirror, as your image usually distorts the further you stand back, so if you're shaving in front of the mirror, for example, therefore very close to it, and also in control of your facial expression, that may explain why it doesn't bother you, compared with in shops. I also think some shop mirrors have a 'magnifying effect', and so combined with harsh lighting, accentuate all of your bumps and flaws!
Just my thoughts:)
Marco.
