OMG is right. I don't even know where to start here
Guys, please at least download my free expressions series. It's free. It will teach you expressions. The design sucks and some of it is a tad out-dated, a little. But, this is all covered in the first two lessons.
The s,s this is simply LOCKING the two dimensions to the same s value. However, arrays are all referenced the same way: array, array, array, etc.
The original question could have been answered like this:
comp("main comp").layer("dolly").transform.scale + [50,50]
Arrays need to add to arrays. You can't just add one. In fact, I'd prefer that AE throws up an error for this.. but instead it nicely adds "50" to your first part of the array and 0 to the the second. That's why you had the values you saw.