I am a Mercury FB collector and in my many searches I have seen some really bad coins get the FB designation from PCGS.
Addition...since the consensus is 63/64/65 (depending), what remains should not be related to wear, but instead is now a strike or handling issue (imo). You can have a strong strike but still not have a completely formed diagonal or split-band. Also, mishandling somewhere along the line could contribute to the flattening of the lines, especially in the very center (from what I have seen on other coins). I began collecting mercs in 1962 from pocket change and have a full range of dates and conditions to compare to. This series is by far my favorite, followed by SLQs then Walkers...Spark