With the ammonia tester, you add a specified number of drops and compare the colour to a chart. With GH and KH testers, you add drops until the colour changes, there is no chart to compare the tube to. Hard water needs more drops than soft water. This does confuse a lot of people.
If your tank water hardness is, say, 16 dH and you remove some water and replace it with the same amount of water that also contains 16 dH the tank hardness will stay the same. This is what happens during the weekly water change, you replace old tank water with new water of the same hardness.
It will only raise GH if you use 16 dH water for topping up without removing water from the tank. In this case water evaporates from the tank but the minerals stay behind. If you then add water containing minerals, the mineral amount will build up and increase GH. For topping up between water changes, ideally you should use pure water such as RO, distilled or rain water.