Let:

- represent one seed;

+ represent two seeds; and let

( and ) both represent a mouth that will eat one seed if there is anything on offer.

On the left-hand side we get:

1 seed, eaten. Another seed, eaten. Two seeds, one eaten, one left. Another added but then that one is eaten. Remainder = one seed

On the right-hand side the action is much quicker:

A wasted bite (cheating here?!). Two seeds, one eaten. Remainder = one seed