battery voltage question

the voltage figures I get are from my meter. The guage is an alt. guage with D to the left and C to the right...I assumed this was voltage that shows charge or discharge

I have a deep cell in my boat that I know is good (checked it cell to cell with my meter) and will pick it up this weekend. The battery in the truck is only 3 yrs old....something makes me think that last year itested the voltage and was the same which made me question it this spring