No, this isn’t a sanitation issue. From time to time, The LEGO Group will make some subtle alterations in the design of certain pieces. The most noticeable change of this sort was when they changed their color palette slightly back in 2004 – more on that in the future. But now there are still changes in parts happening from time to time.
To many builders, it doesn’t matter which mold variation they have – parts are parts, and they will play around to see what they can build. But some folks are planners and they will plan ahead with various design tools to have a detailed set of instructions and a parts list. And in some cases, the parts have functional differences. One of these is the “jumper” plate, a part that enables a stud to be between where the two rows of studs on the layer beneath are (among other things). The newer design has a “stud holder” on the bottom which increases the number of ways it can attach to a part underneath.
When we part out LEGO sets, we have to be aware of the variations in molds that might be there. Sometimes the set inventory will tell you to look for one type, but when you open your box, you’ve got a slightly different piece and will need to adjust inventory accordingly. The differences aren’t always obvious. Consider these arches – there are actually three thickness variants for the top of the arch, we only have two on hand now. I’ve had two different variants even in the same bag of pieces. In this case, I have to look at the part number (on the inside of the arch) to be certain which I’m holding.
BrickLink’s catalogue includes all the variations, and it is expected that your inventory on BrickLink accurately reflects the specific variant parts you have. BrickOwl gives an option for a seller to say “does not distinguish mold variations” (we use this option), but we still have to keep track of the variants so that our BrickLink inventory is accurate. Looking for a specific variant of a part? Drop us a message on either platform and we’ll check to make certain!