Phaser 7800 3rd DPI Value

Does anyone have any insight into what the 3rd DPI values refer to?

I haven't seen a 3rd value in specifications before and they are not listed for any of the Phaser 7800 potential replacements (that I have found).

Is it the height of the print?

Actually I find the figures generally confusing.

Why would photo mode have the lowest DPI except for the last figure?

Asking because I am looking to replace my 7800 at some stage before long and don't want to end up with something with lesser print quality.