This is possible (or the other way around too), but I suspect...

This is possible (or the other way around too), but I suspect the reason is the dose of Carfilzomib the mice can handle is just below the threshold that has a measurable effect on the MM cells. If you have a synergistic combination then you can see effects of the combination even at doses below what can be detected when each is used on their own.

I personally find the massive difference between in the in vitro and in vivo efficacy of carfilzomib and bisantrene to be fascinating. In cell culture carfilzomib is more than 100x more effective than bisantrene, but in the mice bisantrene is vastly more effective.