For the OP....
Most likely the small variations in float voltage you are seeing are the result of intermittent loads (frig, other onboard loads) AND intermittent charging (solar, alternator
On my boat with six golf-cart batteries for the house bank and a high-end smart charger (Victron MultiPlus) programmed to my liking, I see the same sorts of small variations. Even changes in the AC loads
can cause some chargers to "begin all over" or to "reassess" where they are in the charging cycle. For example, when I've been on dockside power for some time with the batteries fully charged and on float, if I switch on a significant AC load (like an air conditioner or heater), the Victron will revert temporarily to absorption voltage.
I'm pretty sure this is what's happening to you, and have the "proof" here in my basement. I brought the six golf-cart batteries home for the winter and they're on 24/7 charge and float at 13.8VDC. There is no load on these batteries, and I've never seen a variation from the 13.8VDC float level I chose. By the way, these six Crown CR-235s are 235AH each and are now 2.5 years old. On this constant float, they absorb a grand total of 4 watts or 0.29A.
Note that manufacturer's charging recommendations vary considerably, even for the same battery from the same manufacturer. I think some are written by sales people, or lawyers, or technical writers or, in rare instances, by engineers. Trojan, for example, gives several prescriptions for charging and floating their flooded batteries. There is no explanation for the variations.
Over many years of use of T-105s, testing of AGMs and gels, and working on client's electrical
systems I've come to the belief that for good quality flooded batteries like Trojans and Crowns a good regime is:
Bulk -- 14.8 VDC
Absorption -- 14.8-15 VDC
Float -- 13.8 VDC
-- 13.2 VDC
Periodic repeat absorption -- 14.8 VDC for 30-60 minutes every 2nd day
- 15.5-16.0VDC (rarely, maybe once or twice a year)
Note that if you provide these voltages the batteries themselves will decide how much amperage they will accept.
Note also that this somewhat aggressive charging regime is in part intended to keep sulfation and stratification at bay, and has been shown in my research
to prolong the life of the batteries.