# u-channel in $gg \rightarrow u\bar{u}$

I've seen that for the QCD process $$gg \rightarrow u\bar{u}$$, where $$g$$ is a gluon and $$u, \bar{u}$$ are the up quark and the corresponding antiquark, there is s, t and u channels.

I perfectly understand the existence of s and t channels but I don't get how is u channel possible. For $$e^+e^- \rightarrow e^+e^-$$ there isn't u channel due to positron and electron are not identical, so why is this channel in the QCD process if quark and antiquark are not identical? Is not the same scheme?