The PS3 Slim actually comes with different PS3 Firmware versions based on the model and region... they all didn't ship simultaneously with the same version.
In addition to your proclaimed 2.85, I have also seen several reports of the PS3 Slim coming with 2.70 (US) and 2.75 (EU - Spain) preloaded.
As far as I'm aware, these (excluding 2.70) were never released as incremental updates by Sony as you can see HERE it jumps right from 2.70 to 2.76 in our archive. Some games may include them on the disc... good luck finding them though.
Of course, PS3 Slims delivered to shops these days likely come with 3.0+ so the woman you spoke with may be correct as well.
Well, if it says 2.85 when you check your PS3 then it is.