At this point in time, there is little info on what the "memories" mean specifically.
I would guess that along with what Asrialys said is a good guess, along with the possibility that it was not trained or even placed in the OT's PC and sat there. Could be that it was breed, then PC'd.
One other thing to consider is he Pokémon's happiness level- it could have fainted a lot with it's previous owner.
Either way, without knowing for certain- this Pokémon did not have much "attention" or a good amount of negative/abandoned "attention, just basing this on the wording you received.
Best answer I have at this time.
edit-
What level did you receive the Venusaur? (32 min lvl)
double edit--
(quick thought after first edit--)
It could also be that the Pokémon's IV Stats are medium/low, Perhaps a reason the OT parted with it-
take the Venusaur to the guy in Kiloude City inside the Pokémon center- he is on the left side- he is the IV Judge- let me know the what he says abut the Venusaur to see if it has medium to low IV's-
just a thought on that is all as a possibility..........