Different EPSG values

Hello All,

I got a geo-TIFF with

Projection: UTM
Datum: WGS84
Zone: 18
The EPSG Code is 32617

I Re-projected the same image to NAD83

Projection: UTM
Datum: NAD83
Zone: 18
The EPSG Code is 26918 (correct)

Does any one know why this discrepancy occurs?
I heard that ESRI not current with EPSG processing or something like that.

Thanks,
Samuel