For the purpose of patrilineal lineage comparison, R-BY39496 and R-FT99418 are entirely consistent. The WikiTree member who added BY39496 as a deepest known terminal SNP doesn't indicate that Big Y or SNP panel tests were taken, but the haplogroup couldn't be identified that deeply in the tree without testing more refined than the 111 STR markers indicated. The member who shows FT99418 took at Big Y-700 test.
In the FTDNA haplotree at present, BY39496 is 13 branches below M207, the basal "R" marker. FT99418 is one of two sub-branches immediately below BY39496.
Glancing at the Pulliam Project at FTDNA that Shirlea mentioned, it shows the actual, tested, deepest SNP of our BY39496 member to be, in fact, FT99418: both WT members are shown in lines 7 and 8 of the project's results. I suspect that FT99418 is a relatively recent branching; it would be automatically updated at the FTDNA project as soon as that new haplotree branch was cataloged, but of course the entry at WikiTree would have to be changed manually from BY39496 to FT99418.
Regarding the STRs shown at the Pulliam Project, they would be evaluated as a genetic distance (GD) of 2. One of those markers is the fast-moving, multi-value marker DYS464. Of the 99 STRs whose mutation rates I've been able to track with at least modest accuracy, DYS464 comes in as the 21st fastest mutating in the "R" haplogroup with a rate of about 0.0033.
By the SNP information alone, I would have expected the two men to be separated by fewer than the 8 generations the Relationship Finder shows. The STR data buffers that somewhat, but the yDNA results do clearly support a patrilineal relationship.
