I have considerable admiration for the whole Sinaiticus project, but especially when it comes to the underlying quality of what is produced. Though one could argue that the web-presence is not as slick as is possible, I think there is little argument as to the quality of the images and the transcription. Everyone who has worked for a long time with a single manuscript develops an understanding for the mechanics of its scribes and correctors. The transcribers of Sinaiticus clearly knew the manuscript.
Take the following example from Philemon 18.
The initial correction is from ελλογα to ελλογι (-γει). Tischendorf notes that corrector C wrote the iota but that this correction was erased (super α C ut videtur ι adscripserat, ut esset ελλογι, sed rursus rasum est). On the website both the corrector who added the iota and who erased it is identified as Ca. Though no reason is given for the second identification it is more likely than not that the transcribers are correct. Ca would normally strike the α out or would mark it in any other way. The fact that there is nothing like this visible, seems to indicate that he corrected his initial correction almost immediately.