What you say is very true, we do assume our friends think like us, but Bingley and Wickham are recent aquaintances. Lizzy prides herself on her discernment, and in Ch 9 she claims she understands Bingley "perfectly." But is Lizzy's discernment as good as she believes? She is wrong about Charlotte--a friend of many years; and it seems she is wrong about Bingley and Darcy, and perhaps other(s).