I think it works like this because Sage's shields are tied to a resource with Toxicon and Scholar's aren't. Granted, I don't think this is a good justification for why a Sage should be able to overwrite more powerful shields, but I think it at least makes sense why it's happening if it's the case.
IMO, let Sage's single target overwrite a Scholar shield but make it where Sage's AoE shielding can't overwrite more powerful shields and that's a fair compromise if this was the intended thinking.