dubious execute/scoreboard/@e interaction

The @e target doesn't always seem to work properly in execute/scoreboard command combinations, as in the following command:
/execute @e ~ ~ ~ scoreboard players add @e[c=1] xyz 1

What I expected to happen was...:
Each entity's objective "xyz" score should increase by 1.

What actually happened was...:
My objective "xyz" score increased by 1 for each entity.

Steps to Reproduce:
1. /scoreboard objectives add xyz dummy
2. /scoreboard objectives setdisplay sidebar xyz
3. /summon Boat
4. /execute @e ~ ~ ~ scoreboard players add @e[c=1] xyz 1

I'd expect each entity to have its "xyz" score increased by 1. Instead, only mine is increased by 2 (or more, if there are other entities around). Note this seems to only affect the scoreboard command; for example:
/execute @e ~ ~ ~ tp @e[c=1] ~ ~ ~-8
teleports each entity 8 blocks north, as expected. Interestingly:
/execute @e ~ ~ ~ scoreboard players add @e[c=1,type=!Player] xyz 1
also works as expected; namely, xyz+1 for all entities except me, +1 additional for the one closest to me.

I know I can work around this by just not using execute:
/scoreboard players add @e xyz 1
but that's only sufficient in this simple example. If I want to use execute's detect clause to only increment the score for entities standing on cobblestone, for instance, I can't do it a single command.