During the champion battle, what's with the rival's Exeggutor?

[WaterTypeStarter]

When you battle the rival right before Victory Road, his Exeggcute has four moves, including Solarbeam, a grass type move. Why then as an Exeggutor does it have only three moves (and no STAB)? No move deleter is in Gen 1, so how could it have an empty moveslot? And why does it have no STAB (meaning of you did not pick Squirtle as your starter, you get no Grass type challenge during the champion battle).

 

[RedBlastoise]

The old games had tons of errors so I'd chalk this one up to Gamefreak not paying attention.

Like how they gave Lance's Dragonite Barrier for some reason even if it was an illegal move for it at the time.

 

[Leonhart]

I seem to recall that Pokemon with limited movesets were a fairly common occurrence in many games, not just in Gen I. I suppose it was mostly an oversight by Game Freak in this case, however. In fact, I often wonder if the rival not having Ratta (Raticate) on his team in the second half of the game was an oversight as well.

 

[Plague von Karma]

RBY trainer battles are set to multiple different types, but are overall glorified wild battles in how movesets and stats are pulled. Gym Leaders and the like are the only times where special moves are distributed to my knowledge, and it's only one, eg. Bubble Beam for Misty. Unlike the others, the champion battle doesn't have any "special moves", so Exeggutor simply uses its base learnset of Stomp, Barrage, and Hypnosis, which was (presumably) designed to prevent Exeggcute from learning anything after evolving while saving memory.

Not sure why Lance's Dragonite had Barrier, but Yellow replaced it with Thunder. It's not even close to Thunder in the move index, so it doesn't seem to be a typo either.

 

[Mewtwo's Amber]

I often wonder if the rival not having Ratta (Raticate) on his team in the second half of the game was an oversight as well.

Why is this an oversight? Why can't he have just boxed it?
Although I find the lore that Red/the player killed the Raticate intriguing, it baffles me how Gary/Blue boasts about having over 40 Pokemon but somehow people seem to believe the box system is a privilege exclusive to the player.

 

[Locormus]

I seem to recall that Pokemon with limited movesets were a fairly common occurrence in many games, not just in Gen I. I suppose it was mostly an oversight by Game Freak in this case, however. In fact, I often wonder if the rival not having Ratta (Raticate) on his team in the second half of the game was an oversight as well.

Or it's simply showing how some early route pokémon lose value to some of the more powerhungry trainers out there. It's a miracle that he still uses Pidgeot.

 

[Venomshock]

Overlooked error I guess? Those older games had less attention to detail than modern ones so of course some strange mistakes can be found.

 

[Shiny Venusaur]

We've seen in newer games that some move sets are shortened to 3 moves to have been AI to pull a better move, but I doubt that was planned in RBY. My guess is similar to Plague's comment - unless specifically coded, the Pokemon probably pulls the moves from the move pool that matches the last 4 level up moves.

 

[KRSplat]

It was said that the trainer released the unevolved pokemon,
and, caught an evolved pokemon of that evolutionary branch:
The meaning being that they were actually 2 diff. pokemon.
I can't verify that this information was canonical though​