grassfed brisket plateau temp different?

So I'm cooking my first brisket and I'm using a 5lb grassfed and finished cut. I'm using, for the first time, a dual-probe Maverick ET-732. When I first turned it on outside in air, both of the probes were reading 99 (the food probe) or 104 (the air/grill probe). I presume this was accurate since I am in Austin, and it was around 2:30pm, and around 100 outside. I went inside and held the grill probe in an AC vent. The intake air according to the thermostat was 77, and the probe was reading 59 after a couple of minutes. This is about what I would get with a candy thermometer up in the AC vent, so again it seems to be pretty close. If anything, the food probe was reading a little higher when outside on the porch (didn't try it in the ac vent), but then again it was in free air.

Anyway, I started the brisket around 3:30 in the egg with the probe showing around 230 degrees F. The dome was saying around 250. Later, the difference between the dome and the grill was much smaller -- when the grill read 220, the dome was reading 235, and around 4 hours now, the grill probe is at 228 and the dome only 4 or 5 degrees higher. I am chalking this up to the temp of the meat causing a slight change in air temperature at the nearby probe when the differential is high.

So to my question: after 4.5 hours, the internal temp probe is reading 154. It has been at 154 for about 1.5 hours now. Is this the plateau? I thought it was supposed to be closer to 170. So I'm wondering if my unit isn't calibrated well, or if a grass-fed brisket just has a different plateau temperature. It is a leaner cut than what I have seen at the local grocery store, for one, and I read an article (playingwithfireandsmoke.blogspot) that mentioned that grass-fed fat is more yellow than grain-fed. OH! it just moved to 156, hmm.. anyone else have experience with grass-fed briskets?