The word translated "highest" is the superlative of άκρος akros:
- At the edge, extreme, beginning, end: outermost (especially of the top)
- Being the most of any characteristic: best, oldest, first
I get the idea that the sense of well-being of the gods is unchanging and constant and so at the "most extreme edge" of the spectrum of how eudaimonia is experienced. Non-gods experience the addition and subtraction (subtraction of pleasures = pain??) in their lives.