Right, with one you're putting the 10 bit value (justified left) into a byte, with the other, you're putting the 10 bit value (justified right) into a byte, trying to stuff 10 bits into a spot for 8 bits.
So, with one, you get the lower 8 bits of the 10 bits, with the other you get the upper 8 bits of the 10 bits.
Which one is it? That's for you to figure out....(hint: if you use a word variable for reading, you'll figure it out quickly if you also use the LCD to display that value)