Something odd going on indeed... Either the bitter oranges were later grafted with lemon plants, or the reason to plant near the lemon tree was in order to prod

Something odd going on indeed... Either the bitter oranges were later grafted with lemon plants, or the reason to plant near the lemon tree was in order to produce, by hybridation, other seeds that had mixed attributes, some of which would be very similar to lemons. The former makes more sense however.