I was told the energy I receive somehow is bought at a different cost - which doesn’t really make sense as it all comes from the same main grid anyway
I wonder if it is due to the energy producers where Bulb buy their energy charges more in the day than the night (when demand is less).?
When demand is higher during the day the producers can charge a premium whereas during the night they may have surplus capacity and hence are happy to sell their energy at a lower price.
The fact you get your energy from the same “grid” doesn’t make any difference it is what Bulb have to pay their energy producers which to some extent determine their tariffs.
Bulb as a green energy supplier has to demonstrate that the energy they supply to their customers is matched by the energy they buy from their suppliers,
This page from Bulb may explain it better: https://bulb.co.uk/generators/