Girl replies on Wall posts on Fb, but rarely on chat.. why ?

[445]

hello everybody, there is a girl I like who I met in a class which got over some time ago. After tht we are friends on Fb. The problem is that she gives nice and big replies on anything I post on her wall., but she rarely replies back on chat.
When I asked her why (on chat) , she said ( a rare chat reply ) that she often drifts away to other sites when logged in , so misses the chat messages often. I find this very strange.

Example :- once I put in a chat to her... the fb seen feature also said that she had "seen" the message.. no reply... but then in 10 minutes time, she replied on a post on her wall by some friend of her.

Can anyone throw light on what's going on ?