I don't have a GPS track of that particular hike, but my experience is that more often than not the distance on the signs in Yosemite are inaccurate. They never add up or make much sense. For example, you'll be standing at Point A where a sign will say "Point B 4.1 miles" and when you get to Point B the sign will say something like "Point A 3.9 miles". And when you get to point C it gets even more confusing since nothing adds up in either direction. Also, <random fact> the sign in Tiltill valley showing its elevation is off by 1000 feet. </random fact> So yeah, I don't trust the signs.
Another thing you can do is drop the track on a map (if you don't have an application that does this use something like everytrail.com) and see if there are any obvious anomalies on your track (if your signal was lost even for a second, which is pretty easy in the valley, your track might show it with some strange spikes or variations). If you're GPS-track savvy you can go in and edit those points out to get a more accurate number.