If I buy a packet of bread:
And put it in the freezer, in it's packet, when I take it out to defrost it tastes different, sometimes drier as well. Why is this? I would of thought moisture would be locked into the packet.
Over the short term moisture is "locked into the packet". As bread freezes moisture from the bread often condenses and freezes on the inside of the packaging. If you then defrost the bread slowly in the same packaging this moisture is reabsorbed. If you only defrost a few slices at a time this moisture is lost when you take those slices out of the bag. These ice particles can also end up in your freezer rather in the bag.
To keep the moisture level closer to the original: freeze bread in portions you are likely to defrost them in and make sure any bags or wrap you use is suitable for the freezer. If you freeze whole loaves double bagging them or wrapping in some way helps.
These steps will also stop other flavours from your freezer being absorbed which should help.
Also, it could be because you are actually RE-freezing it. Many breads come into the stores frozen. The way to tell is in the dating. If the date is printed directly on the packaging or clasp, then the vendor stocked the bread, and it's most likely it was never frozen after baking. If there is a use by sticker placed on the packaging, then it came into the store frozen, and was dated when the store placed it on the floor.
In general freezing any food will cause some degree of structural damage as the water contained in internal structures forms crystals and expands breaking those structures*. Some times the structure was holding the water in place, like the cells in a strawberry and after thawing the water simply leaks out of the holes. I don't know enough about the how water is typically held in packaged breads to say specifically but if the freezing process breaks enough water out of the rest of the bread, than during thawing it is more likely to leave the bread than when the bread was fresh.
*Packaged frozen foods often get around this by freezing food rapidly preventing large crystals from forming.