You say you never mentioned "ROI" but you keep bringing up a figure of 6% without specifying what that is so I'm assuming you're alluding to the annual masternode reward as a ratio of collateral, promoted on Dash websites as "ROI" (which it isn't because the invested capital is dollars or some other currency, not Dash).
Lets take a look at this site :
https://dash-news.de/dashtv/#curr=USD&value=1000
Focus only on three things in this picture :
1000 Dash
4.73 Dash per month
Annual Revenue in Dash : 5.68%
The FIAT info is just additional info, ignore it
1000 Dash
4.73 Dash per month x 12 months = 56,76 Dash
Percentage increase from 1000 Dash to 1056,76 Dash = 5.68 %
Annual Revenue in Dash = 5.68%
Lets take a look at another site that shows a bit higher annual revenue (its is named ROI there)
http://178.254.23.111/~pub/Dash/Dash_Info.html (ROI)

Focus only on the ROI, ignore everything else.
Here the ROI is provided, but not explained. To explain it we need to know the masternode rewards, which we gather from the payments per day :
http://178.254.23.111/~pub/Dash/Dash_Info.html (Payments per Day)

Dash per masternode per day : 0.163939 Dash
Dash per masternode per year = 0.163939 Dash x 365 days = 59,837735 Dash
Percentage increase from 1000 Dash to 1059,837735 Dash = 5.98%
Annual Revenue / ROI in Dash = 5.98%