TextEdit icon thumbnails: Some are "custom icons" that reveal text as it appears in file, while others are "generic" blank white icon/documents. Why?

This has always bothered me and I've been unable to figure out what causes it:

When I create a new document with TextEdit, sometimes the saved file has an icon that features a miniaturized version of what the text looks like in the document, but other times it's just a blank white generic icon with the TextEdit "corner turned down" identifying feature. (All of the documents being referred to here are "plain text," not "Rich Text Format.")

Half my TextEdit documents have the "internal text previewed" custom-icon style, and the other half have the "generic blank white TextEdit icon" style, and I can't figure out why and/or how to permanently change some kind of setting to make all future ones be consistently the same.

To illustrate what I'm talking about, I put two different icons, each of the two different icon styles, in a folder together and then took a screenshot to show how they appear -- both these documents have the exact same content:

What causes these two otherwise-identical documents to appear differently (even though they are together in the same folder)?

Solutions I have investigated:

a. I thought perhaps that somehow the "blank" ones had lost their Finder icons, and the OS was substituting some kind of generic icon, so I did an old trick: In Finder, open the icon's "Info" window, then click on the icon symbol at the upper left of the Info window, and then hit the Delete key. This works in restoring the thumbnail image for jpegs that somehow lost their custom icons -- but it has no effect on these TextEdit files, which stayed as unchanged generic blank icons after my attempts.

b. Searched for some Finder Preference that allows or stipulates for newly-created files to either have or not have "custom icon thumbnail previews" or whatever the terminology would be -- but was unable to find such a preference option.

c. Search TextEdit's preferences to find a way to turn on or off the option to have "custom icon thumbnail previews," but similarly was unsuccessful in discovering such an option.

d. Tried to determine if the icons with internal text previewed all had a different "encoding" (e.g. "UTF-8" or "UTF-16" or "Western [Mac OS Roman]" or whatever) from the generic icons, but could find no way to ascertain what encoding each file even had.

I wish I could tell you under what circumstances the differing file-types were created, because obviously I must keep changing some kind of setting back and forth without really keeping track of my setting changes, but to be honest I'm so frequently and constantly creating new files dozens of times every day (as part of my work) that I've been too busy to note or care about the formats or preferences or whatever. It's only much later, in retrospect, when I'm organizing the files every now and then, that I notice that half are generic and half have textual preview custom icons. I know that at various times in the past either I or one of my colleagues either accidentally or intentionally changed the default encoding of the text files, but I couldn't tell you what it was changed from or to or why the changes were enacted.

This isn't the most significant problem in the world, but it's just mystifying and I thought I might as well get on top of the situation and finally figure out what causes this, and hopefully be able to make all future icons be all consistently the same (doesn't matter to me which way -- customized or generic -- just as long as they are consistent).

10.6.5 on a MacBook Pro, and 10.6.8 on an iMac (both computers exhibit the same behavior).

Any help or insights would be much appreciated. Thanks.