I wonder if you would clarify. You said:

Thanks to everyone here, I figured out that seed starting mix was not the best choice of potting soil for my winter sown seed.

You are using 'seed starting mix' and 'potting soil' as if they serve the same purpose. I have seen posts where many people recommend the use of a sterile seed starting mix for sowing seeds. I myself, have always used one of these starters because of the 'sterile' condition. That way, I do not contaminate the seeds I have purchased or received from a very kind trader. This year, I started with a cheap mix from WalMart which I used for all my winter sowing and I am having good results so far. I used it for some of my pepper plants indoors and they are all packed down and getting mossy, but the seedlings are doing quite well. For my indoor plants I am using a Pro-Mix Prosessional product - rather gritty with no peat or perlite - and my peppers and tomatoes are doing well in that under lights. If we look at nature, seeds germinate under the most demanding conditions,on whatever soil they happen to fall. Germination is a very complex process, involving temperature, humidity, timing, and seed condition, to name a few. Look at all those container we toss only to find they have germinated the next year. I don't think we can base germination rates solely on seed starting mix. There's so much more to it.

I used regular potting soil and have had good germination with a few exceptions.