Maybe a little different than that.
He was a count in the Nordgau, meaning he exercised comital authority in that region, but the exact extent of his rule is unknown and (perhaps) partially undefined. In his case, he likely ruled the whole Nordgau but didn't formally hold the title Count of Nordgau. Instead, he and a cousin were equally heirs of the counts or dukes of Alsace and they had divided it between themselves -- "I'll take the Nordgau, you take the Sundgau, and we'll each keeping trying to grab the other half if we can."
And he was count at Egisheim, meaning he had his seat of power and main residence at the castle there. From Egisheim, he ruled the surrounding territory, the area traditionally ruled from Egisheim, whatever that might have been. In this case, probably the whole Nordgau.
Titles at this period didn't work the same way they did later, with clearly defined territories and specific names. A -gau was an old Germanic administrative unit. Under the Frankish kings each gau was ruled through royal officials called counts. When royal authority weakened, the counts in possession at the time managed to turn their royal appointments into an hereditary power base.
Later, in the process of solidifying their gains they began to standardize their titles. German scholars in the 18th and 19th centuries smoothed out the details, added the regnal numbers, and made a nice, orderly system out of the chaos.
MedLands has chosen this dual form, probably from the surviving records, but both titles really mean the same thing. Secondary sources might call this same guy Count of Lower Alsace, Count of Nordgau, Count of Egisheim, etc. As a 16th century noble, your geography tutor was supposed to teach you these things ;)
This is a very approximate explanation that tries to cover both the generalities and the specifics of this case. I don't doubt that someone else will want to jump in to tell me I've left out some vital detail here or that my explanation is a bit misleading there ;)
