Any comments on this scenario please chaps?

I went to the place where I do the garden, put the lawn mower battery on
my charger, then went off to do other stuff for an hour or two.

Funny thing: when I connected the charger, it stuttered about a bit
showing "charging" (orange light in this case) then "fully charged
(green).

Since the mower's been idle all winter, I knew the battery couldn't
*really* be charged; it wouldn't even cough when I tried the starter
before connecting the charger. And so I left it at green, thinking it
would "sort itself out" -- I was in a hurry to get started on my other
tasks.

After an hour I came back: light on green; disconnected charger; tried
the engine: it coughed, then stopped; no further attempt at coughing:
battery obviously dead.

I connected the charger again and this time it stayed at orange
("charging"), implying that it might get properly charged this time! Had
to leave it though because the mower's not in a place where I can leave
the battery charging and me not there.

Any suggestions as to what happened here? Why did (nay: could) the
charger say "fully charged" when first connected?

The charger is new - a Ring RCB206 Automotive Dual Voltage Standard
Charge 6, 6amp, 6 or 12 volt (used it on 12 volt for the mower).

Any thoughts appreciated!
Cheers
John