Is it consistent to say "I know that London is the capital of England, but I am not certain"? For I could have been lied to all my life, so I am not certain of it.
I can know that p without being certain that p if, for instance, I have evidence sufficient for knowledge that p but am uncertain whether the evidence is sufficient. No contradiction.
I can also know that p without being certain that p if, for instance, I knew that p at time t1, I remember that p at time t2 but, at t2, am uncertain whether I am veridically or falsely remembering. No contradiction.
The avowal, 'I know that p but I am not certain that p', raises different issues. In saying, 'I know that p' you imply that you have the highest degree of confidence that p. In which case how can you consistently avow also that you are not certain that p, since not being certain that p implies having less that the highest degree of confidence that p? Contradiction.