Mike:

Way too many variables at work here.  Did the slot not close because it was wetter than nearby soil?  -- i.e., a tiny bit of a swale, which would also make it cooler, more prone to pathogen problems, less N available, poorly aerated, etc.  Does the soil texture in that spot cause it to hold more water? -- i.e., higher clay content, etc., which would again cause the above problems, but would also change chemical characteristics such as nutrient availability.  Did it not close because of heavier mulch cover?  -- in which case, it would be colder, wetter, far fewer nutrients available, etc.

Myriad of possibilities.  I just don't think you can hang the entire effect on lack of furrow closing.  Need a more controlled experiment to sort that part out.