In theory, the oval ring takes up an equal amount of chain all through its revolution. That's all fine, but given its shape, the top (and bottom) point where the chain exits/enters the ring sorta moves up and down. (That's the whole point of biopace, the changing leverage). That means that the chain goes to the cog at a different angle, from a different point vertically. Obviously, it would take a longer bit of chain to connect from the cog to a point higher up on the upper section (and lower down on the lower section) of the chain loop... so it changes tension a bit.

BTW, this is why the "add 4 teeth to the chainring and you will be fine with one more link in the chain" axiom doesn't always work with vertical dropout frames.