Dynamic? 

My carrier performance is excellent. You can see my delivery ticks on sales and how quickly they are being delivered. Historically my issue has been eBay making the estimates too optimistic and thankfully I have avoided late deliveries by the speed of Royal Mail's service. 

My handling time is fast, usually same day. I only changed it to 2 day dispatch to try and offset eBay's optimism.

The buyers location seems to make no difference in my case. I have checked this by asking friends who live local to me and further afield in the UK what they are seeing on my listings and it is the same.

I also have a private account and this EDD issue hasn't affected that account, which is strange don't you think?

I appreciate your response but it seems like a slap in the face. If 10 day delivery estimates to buyers on an account with no late delivery history or local carrier issues isn't a glitch then I don't know what is.