> First, when I hit `C-s ea' I see 1248/2520, but then another
> `C-s' shows 1249/2530 - the total has jumped by 10.
>
> I continue searching forward, and the numbering seems normal:
> 1250/2530, 1251/2530.  OK, so the initial total was off by 10.
>
> But at match #1251 I switch to `C-r', and the match number is
> 1241/2520: both the match number and the total are off by 10.
>
> On a small buffer I don't notice this problem.
>
> Do you see this too?  If not, maybe I patched mistakenly.

I see this too.  The problem was in too frequent updating
of the Isearch prompt - the loop updated it after every
max-at-a-time (default 20) matches, thus causing flickering
(that didn't update the final number).  Now changed to show
only the final number without showing intermediate numbers
(that are invalid during counting anyway).

Please try a new patch, it removes flickering: