The answer is actually relatively straight forward that you started to lead onto yourself.
It really comes down to the level of testing the individual has done, and further testing may actually further subdivide subgroup 8 as specific SNPs are identified to categorize them.
Note the Y-111 test is a Y-STR test, not an Y-SNP test. You need SNP testing to get more granular and down the Haplogroup tree which goes quite deep. The YTree gets updated monthly on YFull based on people performing BigYs and uploading them to YFull.com. FamilyTreeDNA only updates their YFull tree about quarterly or every 6 months (no official schedule)
So those that have something beyond R-M269 have done one of three tests.
- FamilyTree DNA Big-Y
- FamilyTreeDNA SNP Pack
- A National Genographic 2.0+ Test; processed by FamilyTreeDNA.com
Note: SNP Packs are not available for all branches of all Haplogroups.
The members of sub-group 8 belong to the following:
- R-M269 (STR test only, Y37, Y67, Y111)
- R-L21 (SNP test required, SNP Pack or National Genogrpahic)
- R-DF27 (SNP test required, SNP Pack or National Genographic)
- R-Z16944 (Big Y Test Required for this level of detail)
Note: For new branches or for those who have done BigYs but FamilyTreeDNA's YTree is out of date FamilyTreeDNA defaults to the main branch like R-L21 or R-DF27.
These results tell me the Sub-Group 8 will be further sub-divided as R-L21 and R-DF27 are different branches of R-M269 and those individuals are likely not as closely related unless the mutations are on fast mutation markers of the Y111 test. I have seen a difference of 2 occur in a single generation or even between two brothers on these fast mutating markers and one brother.
The DYS504 is one of the faster mutating markers, and if the kit marked R-M269 did an SNP test they would likely be in the same sub-clad or one under R-L21.
Per the R1B FamilyTreeDNA Project, each also having their own project.
"The largest subclades projects are the P312, U106, L21, DF27, U152, and R1b basal (M343+ P312- U106- early branches) projects."
The R-Z### branch that likely doesn't currently belong in either of those branches but I can't currently find that SNP on the Yfull tree but think it might fall under R-DF27 based on the naming convention.
You can also check out the R1B Project on FamilyTreeDNA and the R1b and each of the Subgroups on YFull.com.
If you do get a BigY, I personally recommend you paying the extra $50 and transferring it to YFull as well because you will get a much deeper interpretation that FamilyTreeDNA is currently able to provide.