Well, let me try my wits to explain this.
1. -10 outside at about 4 pm likely means around -18 at 4 am, so even accounting for the greenhouse, your rye will still suffer from cold damage. And crops cannot grow below 0, whether rye or pineapple, the threshold is just there to protect from cold damage, not from halting growth.
2. The difference could be due to the game simulating the temperature block by block or just some delay in the simulation(similarly, the greenhouse takes some time to validate every block's +5 temperature). With a long enough period, from my observation, every crop's growth will stunt under occasionally too low ambient temperature.