When you zoom in your making a pixel bigger (same effect will happen if you scaled a low resolution image)
When you scale something like text (vector art) or a huge image (that has been resized to fit the canvas) you are nit streching pixels. For vector art the program will draw on new pixels to infinity, formlarge images you are able to stretch (scale) them up to 100% of the original size.
Biggest misconseption with noobies is that pixels (resolution) is mesurable like an inch, its not.