Yes you are right. Mark is also a human being. Every day is different, also his mood.
Mark autographed my french Promo-Maxi-Single Telegraph Road and he recognized the other signature of John Illsley on it. So on that day it was not a big problem to sign other earlier autographed items. In Cologne i have met another fan with a Squier-Guitar filled with autographs by other former Dire Straits members incl. David, Pick, John, Alan, Guy. He also got Mark´s signature on the guitar-body.
You cannot predict it, if Mark or another artist is in a good mood for signing items for fans.
A little story by myself:
In Oberhausen at the 2011 MK and BD-Show i was the one and only person at the stage-gate, who´s waiting for a chance of getting items signed. THE ONE AND ONLY, just remember. That was also a very remarkable situation. I had very good smalltalk with the security-guy at the gate and he informed me, when the busses and cars came or leave. After half and one hour of waiting and hoping, the gate opened and a big guy came to me, directly. It was Pete Mackay, the well-known second tour-manager/assistant-tourmanager of Mark.
He came to me and he invited me to follow him into the "sacred" backstage-area, where all those tour-busses and other tour-vehicles had beeing parked. I gave him my Vinyl-Version of KTGC and my black marker. He walked away into a small building close to the big stage-doors at Oberhausen-Arena. From the distance through a big window i saw Mark and Pete together talking. A few minutes later, Pete came back and gave me my autographed Vinyl back. So thank you again Pete for that cool view into the backstage-area. Than he told me, that i had to leave that area. He made my day, anyway. It was a very kind and positive experience and no other fan was there.