When UO first came out, there were no such thing as seeds or a plant system.
There were, and still are, fishing weights which are usually located inside an npc fisherman's guild or a shipwright. They use the same graphic as a seed. Actually, it's the seed that uses the same graphic as the fishing weight. The fishing weight came first.
Stop by an npc fisherman's guild, and you'll probably even see a fishing weight on display.
Now, when the plant system was first introduced, the developers and/or artists took the fishing weight graphic and decided to use that as a seed graphic. All they have to do is change a few codes here and there, add a couple new lines of codes, and now you've got a new item: a seed.
It's pretty commonplace in UO to re-use graphics either by re-naming them, re-coloring them, or both.
Sometime in the past, someone either found a loophole in the system, used an exploit, or took advantage of a bug that was able to convert a real, plantable seed into a genuine fishing weight which serves no purpose except for displaying.
The seed isn't just re-named, the server actually re-coded it to its original graphic of a fishing weight.
I don't know how or when it was done. It could have been something simple such as selling a seed to an npc fisherman and buying it back from them, trasforming it into a fishing weight in the process.
Needless to say, the method used to create the fishing weight was never intended, and it was fixed a long time ago.
That, in turn, makes it a bug and a rare item, even if it isn't highly sought after.