How come sometimes I get a dry airy sound coming out of my sax?

You probably have too open of a facing on your mouthpiece or too hard of a reed, or both. Airy sound is good for Jazz.

or its` the sax

If the dry sound occurs when you first start playing and passes after playing for a while, it may be a dry reed. Dry sucking on the reed longer before playing.

Try (not dry sucking) rather... I was in a hurry while typing this.