I did a little digging and they show that all 3 cards were released at the same times April 2000 however both the Eevee and Porygon say April 2000 through Dec 2002, where the Karp only says “April 2000 onwards” maybe the Karp release didn’t last as long and this is why we are seeing these results
@crystalcollector149, @cullers, I’m quoting tradenatural here, he said he had the magazines, in the first magazine prizes were the FC Eevee (500pts), a playmat (600pts), and Porygon (700pts). And later they included the karp for 600 pts insetad of the playmat. That’s why it’s rarer.
If you have a complete set of trainer magazines you can check them out.
Before Vol. 12 (July 2001 issue), there was no FC Shining Magikarp at 600pts, only the official Playmat. I don’t know when they stopped giving out karps because I do not have the complete set and my lastest issue is Vol. 12. I’m not even sure if it has already been given out in Vol. 11 but from Vol. 10 and before there was no FC Karp available at 600pts.
What I do know is that the FC Karp is given later than the Eevee and Porygon.