Back in 2012 I just blamed it on him not working on his vocals during off-tour times since he also had sounded rather average for the promo and one-off shows in 2009 and it had even taken him up to 15 shows in 2010 to get back into the groove.
But looking back at it - the new-found way of singing had probably also taken its toll on him. Not only did they tour immensely between 2008 and 2011, but as he regained confidence in his singing they dug out all of the vocally demanding stuff that had mostly been avoided in the 2003-2006 period.
He was in good form in Europe 2011 and it was a great time to be a fan, but there were already some signs of him being on the maximum of his capabilities. In Dresden and Munich I noticed him straining his chords much harder on songs like IBTFY than he had done in 2010, in Barcelona the energy was immense, but the voice not up for all of the set and Dusseldorf had been quite off for most of the night.
While he was still able to hit a great amount of notes and was stunning on nights when he was on (like I attended at the concert in Mannheim), his tone had slowly degraded (to an even more nasal one) from 3 years prior. I always like to take I Believe for comparison: Yes, the Lisbon 2011 version was stunning in its way of delivery and the passion behind it, but going by the singing alone the one from Twickenham 2008 is much easier to listen since he didn't push that much to belt out the notes. It seemed so much more natural then. That was moaning on a high level for a 50 year old who had to sing these difficult songs back then, but a difference had already been noticeable.