Why Fred and Geaorge couldn't share?

[monzaba at poczta.onet.pl]

No: HPFGUIDX 22715

I was re-reading CoS and came accross a thing I haven't noticed 
before, but now cannot understand. 
When they're talking about buying all those lousy Lockhart's books, 
George says: "Dunno how Mum and Dad are going to afford all our shool 
stuff this year. Five sets of Lockhart's books!" Why couldn't they 
share? Why not buy one set for them all - or perhaps one set for the 
boys and another one for Ginny, as they didn't know which house she'd 
end up in? 
And certainly, Fred and George could have shared their books, because 
they most propably were to do their homework together anyway! 
Sure, it would't look nice, if all other students *had* their own 
copies, but surely the Weasley twins could have explained this away? 
Saying, for example, that they were taking too many joke things to 
leave room enough for the Lockhart's stuff in their trunks.
Any thoughts?

Monika
(The Snapefan)