No look closely at the first and last frames, the level in Wally's cup and the coffee pot don't change. This may give us a hint for Wally's unusual pouring technique.
Wally is a human/coffee hybrid. He doesn't consume coffee like us humans do, he has his own coffee cycle. He refreshes his cup at the coffee pot. This involves taking on fresh nutrients into his coffee, and expelling toxins back into the pot. Too bad for Dilbert and the rest of the crew.
I suspect there must be a coffee/human hybrid at my workplace also, as this would explain why the coffee always tastes like $#%!.