BIAB strike temp off by about 4 degrees (low)?

Help please; I think I must be missing some setup adjustment I should be making.
After utilizing the very good info on adjusting efficiencies (also previously a problem; no longer) that came up when searching for when I'm asking after here, I still didn't locate what I know I must be missing.

Here's the issue: I do BIAB in a keggle.
Using today's results for details: strike water in the vessel preheated to 161 (ish) when BS2 says I should be at 158 to hit proper mash temp of 153.
However, after I mashed in, I found the actual temp to be 152.5, 1/2 a degree low despite being 3 degrees high on strike temp.

This is just using current data for a repeated problem; I never seem to be able to rely on the temp values BS2 says I should use in the mash tab.

What do I need to adjust?
(I've got the vessel material specific gravity set a tad higher than the stainless level suggested in the equipment profile, but if I've got "adjust for equipment" unchecked, it shouldn't matter anyway, right?!)

Thanks for help!
Nate

Things to check:

Make sure the grain temperature is entered into BS2 taken just before you reach mashing temperature,

Since you BIAB, you do want to make sure that 'adjust for temperature' is checked off.  Otherwise, you need to enter the mashing temperature in as your equipment temperature in order to make the calculation correct. 

Make sure you have the weight of your keggle correct.

Lastly, you can adjust the thermal coefficient as needed to have the program predict your actual results.  Remember that this is modeling software and needs to be tweaked to match your particular equipment and process.