The reward calculation is actually quite complex and I doubt I can sum it all up in a single comment but I'll try to give you a gist on the points you mentioned:
First, the 1000HP is not a fixed value and amongst a couple of other things it mostly depends heavily on the market value of the HIVE tokens... if the price of HIVE rises, the required HP to generate $0.020 will drop accordingly.
Second, that's where it gets complicated, rewards are not adding up on a linear scale, but on a so-called "convergent-linear-curve"... it's almost linear, but not quite, especially not in the lower range of rewards. The bunny posted a lengthy article about that when it was introduced in HF21.
I've not even paid attention to the value myself
Way to go! And eventually it'll pay off :D