Why is it two drops in mixing bottles and in a glass of water – surely the person taking the glass of water will get more remedy?

This is true, but the amount of remedy is not important as long as the minimum dose is taken. The minimum dose is the amount you get if you take four drops from a mixing bottle.

When putting the remedies in a glass of water, then, you are probably taking more than you need, but it gives you a margin for error. You can sip from the glass without worrying about how big the glass is or how much water is in it or how much of the water you have drunk, because even one sip from the largest glass will give you the minimum dose.