However, I have just talked to http://www.uesp.net/wiki/Skyrim:Cedran, the reachman hostler for Markarth, and he said it was Igmund's father, and that Igmund is quite young for a Jarl, especially of a major hold like the Reach.
Is it stated elsewhere explicitly that it was Igmund and we have a case of "left writing had not knowing what the right writing hand was doing"? Or did we just assume "Jarl of Markarth = Igmund"? At the time, Ulfric was not Jarl of Windhelm either, his father was: he states that he had to write his eulogy for his father's funeral from the jail cell after being arrested for the incident and have it smuggled out or something like that.