What is she on about??

Me and my proper best friend were just talking well talking with signs so no-one would know what we were on about-we were talking about someone fancying someone. When our newest friend (ill call her H) said what are you on about. i replied nothing just...then my best friend completed my sentence for me. Later the three of us were walking down the corridor when my best friend said something and we both burst out laughing. H just looked at us. Then when we were all waling home i was winding up my best friend by saying someone on this movie died (i already seen it, she had only seen it half way through). But when she found out H just said "you too are like ant and dec you know that?" but we just said this at the same time "no we are Laura and Georgina!" what is H going on about she says weird stuff like that all the time. Shes my friend and i woudnt change her for the world but it would be nice to know where she gets this stuff from. Anyone offer an explanation?