Delivering branding to a client

For some time now most clients I'v dealt with are rather pleased with my method of deliverables. I'll deliver the logo as an AI, jpg, PSD and an png. But my most recent client is 'disappointed' due to the fact they actually wanted and ESP. Now I understand what an ESP is, but why would they ask for that specifically?

I'v delivered what they need and require but personally I think it's a 'nit-pick'.