After doing this a couple of times I noticed that for every card I hide it decreases the total number of cards in My Collection.
A piece of history - a few customers, for reasons that I can not understand, requested that there be a way for them to completely remove cards from their collection as if they had never bought them, never to be reminded that they had bought them and never to see them offered for sale or won in the games so "unbuying" them as they initially requested would not have worked.
The hide function seems to have been provided to satisfy this request and this is probably why the card count decreases and why it takes a litle effort to recover them (otherwise a simple addition to the filters would have sufficed).