Leaking valves will cause compression to read a lot lower. As the rpms increase, they will effectively start to seal better as there is less time for leakage past the seat.
I have seen( wanna pic?) valves with pie-cut shaped wear, and with slices missing that have zero compression.
If you are lucky, the valves can be ground, the seats cleaned up, and it will work better. Check the supports/lifters for those cylinders for sure. Make sure you can compress the lifter as a frozen lifter will (can) keep the valve from seating and it will cook again in no time.(well... some time) If you cannot compress the lifters, take them out, and disassemble & clean. Re-assemble using the same parts(do not mix!!! !!! !!!) and put back into the same place they came from. Check also that the valve springs have some strength, as springs that have gotten hot may lose their temper, and be weak. Weak valves will not seat tightly, and cook as above, and may not work well at higher rpms.
Something caused these to burn. If the cooling system has been neglected, now is a good time to remove the pump, and flush from the rear coolant passage holes in the top of the block. Any sludge and stuff will have an egress port through the pump hole.
IF you do that, make sure to dry the cylinder walls after. You will likely be unable to keep water/coolant from getting into the cylinders atop the pistons. You can use rags or paper towels to sop up the coolant, and then rotate the engine slowly, by hand and examine the cylinder walls for droplets, wipe, repeat until you don't get any drops after rotation. You can also use a rag/paper towel wetted with engine oil to wipe the cylinder walls and prevent flash rustification.(rust)
tom