Not really sure when I noticed this for the first time, possibly as long ago as 6 months.
When I go to the Galactic Market there is a summary for each asset type, just above where you can Checkout. Usually there's only items available, most notably KeyCards. The number of items in this summary is seldom the same as the number you get if you click on "items" in the summary, today the difference is 13, earlier it has been 17.
Summary (at the time of this writing): 359
Items Listed on the Market (at the time of this writing): 346
Does the fact that I'm not in a Commerce Centre, Trading Station or Centrepoint Station have any relevance in this case? Or is there another explanation?