We can never know for sure unless we have all information, but my guess would be that the difference in progression between you two was caused by your decay equal to 20% of your total RP, while your friend's was only 10% since he was below R7. This is twice as much and it seems to fits the situation you described.
Regarding your Standing, it seems indeed fine for R8 (altough I don't know but eh, if we have like 2-3k ranked players in each faction and WS 102 isn't enough to get to R8, R14 might be impossible ^^), which means it's enough to beat decay every week and thus if you get the same WS everyweek, assuming a constant number of ranked players in your faction, it will grant you progression each week and will allow you to reach R8.
But it doesn't guarantee a to attain a certain rank a given week. In the meantime, if you want to progress faster, you just need a better standing.
I know there are some assumptions, but that what it looks like to me, hope this helps
