Can't say it applies for you, but often the outbound "upgrade" calls are a little bit before a contract ends. If the punter takes the bait, they'll be accepting the first deal VM offer, probably won't have had chance to research other options, and are then locked in for another 18 months on a deal that may be poor value (although a few VM deal offers are pretty good value). If VM allow things to drift rather than encouraging people to re-contract before their current fixed term ends, the punter usually waits until the contract is about to, or has just expired, and makes an inbound call to speak with retentions, and in that situation the customer has much more leverage, and the time and opportunity to research alternative offers, meaning VM have to offer a deal that satisfies the customer on bundle and price, rather than the bundle VM want to sell at the price VM want to sell it at.
In terms of the 100 > 500 there's a logic (for VM). On a 100 Mbps connection you'd notice little difference on a good 60-80 Mbps Openreach line, so leaving VM has limited disadvantage. If they've got you on 500, then VM know full well that many customers will be loathe to downgrade even if they're not using the bandwidth they have, and next time round the retention pitch will centre on "well, we offer far better speeds that you won't get elsewhere, and you'd struggle if you've got more than a single adult and a cat using a 60-80 connection". This argument obviously won't be effective to the lucky 5m households who have Openreach FTTP available so far, and the 20m who will be joining them over the next four years, but for now it plays quite well for VM.
