Most Helpful Guy
If a guy is willing to tell you this kinda crap I hope to god you're not looking for a long term relationship with him. Anyways to answer your question I have no idea maybe he likes to be honest with you because he has been honest compared to her he's likely been lying trying to impress her and doesn't know how she'd act to honesty? Maybe he likes her more and doesn't know if she's really interested and doesn't want to ruin it? Maybe he's trying to get a reaction out of you?
Most Helpful Girl
It sounds like he started seeing her first. It seems who ever comes along second always gets the news of the first person they are with, and not the other way around. People fear being alone and knowing that they are being deceitful will likely run the risk of them losing their "primary relationship", which is usually Relationship #1.
Anyway, the bottom line is you are currently with someone being deceitful and is practicing their lying skills while with you. I'd take this very slowly and not get your heart wrapped up too much until he can get rid of the other girl and just focus on you. Good luck! :)