In this sentence "deviate" is used in the sense of "not conforming to something". If the behavior of a real gas doesn't conform to the behavior of an ideal gas, their behaviors don't match up - their behaviors are different.
Here's another version of the same sentence,
"There's more of a difference between the behaviors of a real gas and an ideal gas at lower temperatures."
Also, you need the word "you" in your question: "Could you explain it to me?"