When they were recording FANSD
and it came time to record Matt's vocals, he got sick and was unable to record them in the studio during the alotted time they had to record them. So they ended up using "scratch tracks" which, from what I gather, are the vocals singers will sing live with the band to help guide the recording of the rest of the album instrumentation. They're basically demo vocals--and Matt's vocals on FANSD
in this case were used from those demo tracks in place of him recording studio vocals. The band was pleasantly surprised at how good Matt's live, scratch vocals were, so they used them in place of legit studio recorded vocals (multiple takes, etc.). This was documented in at least one or two interviews around when the album came out. Fun fact.
(And someone else responded about this already, I just saw, but whatever).