That has to do with the way the system rounds to percentages. The 1% drop in health does not effect actual HP (located above the four move choices), it's just a graphical thing. Same thing with your percentage problem when attacking. For example, if Scyther had 300 HP, 150 HP would be 50%. However, if you deal 149 HP worth of damage, PS! rounds that up to 50% health lost, while rounding up the 1 HP remaining to 1% health remaining. Again, these things are purely graphical. They do not have an effect on the actual HP totals on the Pokemon whatsoever, meaning they do not impact the outcome of the battle at all. As such, they're low on the list of problems to fix, I'm sure. There's more important stuff to do.
: Updated 4/27/12
&V4: Im clam phoopes
&V4: IM FUCKING CALM
Aurora: why don't i get any action