What you are describing with words and pictures is the loss of plaster “creme” in the areas between the pebbles. Loss of cementitious material will happen either by acid washing the plaster OR if the pool water is aggressive (low saturation index) for long periods of time. It is also due to a natural variation in the plaster mix as it was applied to the pool. The original plaster applicator might not have been careful with keeping the plaster pump properly filled with mix and they may have overdone the pebbles a bit. Improper troweling at the time of plaster application can also affect the ratio of pebble exposure to plaster creme. What you are seeing is the natural density variation of pebble stones on the mix. At 16 years, it’s not at all unusual to see that.
How are you adding acid to the pool to control pH? Are you keeping a good watch over your saturation balance so that your water doesn’t become aggressive towards plaster?
As for fixing it, you can’t. No plaster company would chip out areas and patch as that would look worse than what you have right now. Your pool needs to be replastered soon (within the next 4 years or so). Start saving up and think about how you would like to remodel the pool. If you don’t plan to be at that house in the next 5 years, then leave it alone and let the next owner deal with it.