Why does the HamQSL website say the 40m band is fair and the 20m band is poor during the day?

The "HF Conditions" indicate the general, worldwide, relative propagation to be expected from the bands during your station's local day or night. I wrote to Paul, N0NBH, for his response to your question, which he agreed could be posted here:

Thanks for the email and question. I’ll do my best to answer it, but have to remind all using these that its not an exact science. It’s a prediction based on available data and a most likely scenario.

For HF predictions I use what’s available to predict the band conditions, given the obvious night vs day differences (given SFI does not differentiate, its 3 measurements in Canada). Also note I combined 80-40, 30-20, 17-15 and 12-10 to simplify, but that adds a level of error in the calculations. And of course the dayline, short and long hops, etc all are impacted but not in the predictions.

For HF predictions I use K-Index, Solar Wind, X-Ray, and Solar Flux reading for the predictions. In your example of 20 and 40, you are right the D-Layer impacts 80-40 meters the most, but the primary factor for D-Layer Ionization is X-Ray (Hard). And lately its been <A1.0 or non existent. And at Night the D-Layer goes away, hence the difference between day and night. That leaves only the F Layer at Night to impact 20m, but during the day both E and F2 impact that band. That’s where the SFI reading most impacts the band. I created the following slide to try and put all together. Hope it helps.

Again, I am open and honest about the predictions that are based on available data and a likely scenario BASED ON DATA ALONE, but I always recommend the only way to know for sure is to turn on the rig and call CQ!