I can't really make sense of it to be honest. I agree with most of what's been said so far.
I don't think it's totally random though - I feel static parts of a page (CSS, js, HTML) and notably things from CDNs suffer from the issue far more if you're watching in developer tools. This is why I've suggested it's some kind of caching proxy that just isn't performing sometimes. Popular sites seem to suffer far less. If you're googling something obscure and looking at a lot of random sites, it's painful.
But then again, I didn't see the issue on another mast further down the road... So perhaps more like jchaimer's theory.
I did raise this with Smarty and requested that they raise it with their network team. After determining that I was using no data (because they were looking at the wrong mast), I've not heard from them since.
I no longer actually need the connection now so I've not been using it since the weekend. I don't plan to use it or chase Smarty now.