“Seminars” reward 4 EVs for every hour the Pokemon is assigned, or 96 EVs for 24 hours.
So the EV's you got for Speed was correct. But,
Notably, the amount of EVs earned from these sessions is affected by whether the Pokemon has a Power Item equipped, or is infected with Pokerus. To use the Attack Seminar as an example:
1 Day Attack Seminar with Pokerus: +128 Attack EVs
1 Day Attack Seminar with Power Bracer: +288 Attack EVs (Maxed Out)
1 Day Attack Seminar with Pokerus AND Power Bracer: +432 Attack EVs (Maxed Out)
So, as your Volcarona was holding a Power Lens, the amount of SpA EV's it was supposed to receive became 288, but as 252 is the Maximum EV's it can have in a stat, it received 252 EV's.
Hope I Helped