Your meter is reading it wrong. Looking at the photos of the converter I do not see anything that looks like a filter cap. (There is a small yellow thing that might supress RFI a bit but no way is it big enough to do serious filtering) thus supply is putting a mixed AC/DC with an average value of around 13 volts, but your meter is reading the peaks.
I would suggest replacing with a Progressive Dynamics 9230, This one has filtering, it is also a switching power supply, it is a 3-stage battery charger as well.. NOTE: 9230 is the proper size for one Group 27/29 batteries. Larger battery banks can use larger converters.
Randy, Best converters, he is the man to talk from
Congrats on being able to take good photos and voltage readings though.. Dang few here can do that.