I like TMX, but don't use either that or TMY much any more. When I did use them I found the same thing. It is not all that likely that Kodak made a mistake on this. There were some erroneous times given, but not usually on something as basic as this.
Originally Posted by Joerg Bergs
I think it is more likely that the very different highlight contrast of the two films is the reason. I shoot a fair bit in full sun and in available indoor light. In those sorts of high contrast situations I think the very slight tapering of the highlights with TMX compared to the straight line (or sometimes slightly upswept) highlight curve with TMY means you can give more development with TMX (or need to give less with TMY, depending on how you want to look at it).