The Buffalo Bills play just two linebackers on defense the vast majority of the time. So, while their “base” defense might be a 4-3 front in theory, it’s really a 4-2, or a nickel defense, in practice. The third linebacker, while important, is a reserve in this defense, so he’s not as valuable as the third corner is to the scheme.
This means that Buffalo has the option to carry players who excel on special teams at the linebacker spot with most of their reserves. Generally, Buffalo carries six linebackers, and two of the six have exclusively played special teams in years past.