I'm not sure about the situation where you live, but many of the shops around here don't have a huge selection of mountain bikes right now. They may have just had you try those because they had them in stock. Not to say that you couldn't make those bikes fit, but if you have to make a number of adjustments you'll have more flexibility if you're closer to the middle of the bike's size range.

It would suck if you ended up with a bike that you never felt comfortable on and missed out on something as fun as mountain biking