The one thing that I admire about dull with all his faults(and there are countless btw big LOL) is that he doesn't blow opportunities/clear openings and this is where he scores over Fed.
When dull has a clear shot, he rarely ever blows it. He either gets roasted cause he just sucks(surface/opponent whatever) or he's injured(let's just say the injury is legit for the sake of argument).
You will never see dull lose from a position of victory like Fed did at AO 05, USO 09, USO 11,15 etc etc esp AO 05 and USO 09.
I mean yes dull lost AO 14 when he didn't have to face DJ, but he was blown away by Stan. No chance for dull, IDC what dulltards excuse it as.
Also AO 12, he was unclutch for a brief moment which cost him the match but he should've lost it in 4 and was up against DJ on his best surface so I wouldn't call it a CLEAR lost opportunity.
dull has never had an AO 05/ USO 09 type of defeat that Fed has and that is why he's only 3 slams behind.
At the end of the day, if dull does indeed surpass Fed's slam count which will be a total catastrophe, Fed will rue the missed opportunities like AO 05, USO 09, which were clear openings that he simply blew. Fed has only himself to blame for not being at 20+ slams right now.