I will guess it is a Comp Style Amp, X Force/FatBoy/DaveMade.
they want you to keep reflect low because they know that most of these amps will be abused.
2879`s can ( in a properly built amp )work into a 2:1 swr all day long provided you do not abuse the transistors.
When you start to drive them hard, and increase the voltage supply to 18/19/20+ volts that is out the window.
Under normal conditions, using just a swr meter what you are getting now is fine, even if you do get it to match better it will not change anything as far as TX/RX is concerned.
Now, if you do want to go ahead and see if you can get it better........
See if you can you loosen the set screw and raise the stinger just about a 1/4 inch and check 1/20 and 40 again.
Use a sharpie and make a mark were the stinger is now, and then see if you have any room for adjustment.
The Ideal tune would be 1.0 at channel 20 and a slight, even rise on both ends.
Your antenna now shows a little better match on channel 40, this tells us that the antenna is just a little short, and if you add a bit of length it should drop slightly.
Just don`t let it get you twisted if you can not get 1:1 across the board.....
73
Jeff