True I believe I mentioned this a few chapters back but the only reason I think Itachi is fighting Sasuke is because his eyes have deteriorated to the point where he doesn't have a choice but to take Sasuke's now whether he has MS or not.

As far as Sasuke seemingly owning Itachi while he struggled with Deidara I suppose the big difference is that with Deidara that was a physical battle where as with Itachi its a mental battle. Much like when Sasuke fought Oro he lost the physical battle but when it came right down to sheer will power he was able to overcome him.

Well I'm looking forward to the next chapter hopefully we will get a proper explanation there, at the very least we will see Itachi get serious for the first time.