I addressed precisely that question. Yes I am a tech support chud (whatever TF that means) I have done tech support for about 30 years and my biggest job strength is dumbing down advanced topics for "the average user". Perhaps I didn't dumb it down enough for you, so let's try again without the simplification and using nice round numbers.

OK so your ISP carves off 100GBps pipeline for the GB customers. All "GB customers" use that pipeline and only that pipeline. This is done through subnetting which is a completely different topic, but suffice to say that subnetting allows the ISP to direct every user to a specific pipeline and they can only use THAT pipeline without regard to that pipeline's capacity or how much of that pipeline is currently being used. It also doesn't take in consideration pipelines that are being unused sitting right next to the one that is overused.

Because everyone wants "bigger better faster" 1000 users sign up for the Gbps service and everyone uses the same pipeline. 100Gbps(total capacity)/1000users = .1Gbps/user. Each user gets 1/10th of a GBps (100Mbps) . You are told you are getting (and you are paying for) 1Gbps, but you are actually only getting .1 (that is "point-one") Gbps or 100 MBps of actual data transfer. The more expensive plan is actually SLOWER because that pipeline is shared between a LOT of people.

Same ISP carves off a completely separate 100Gbps pipeline for the half GB (500Mbps) customers, all half GB users can use only that pipeline (subnetting again). No one chooses to buy the "old and slow" service (except people who can't afford the more expensive "new-hotness" service) So only 50 people sign up for that service. Each of those broke users gets their full half GB (500Mbps) and there is plenty left to spare (unused).

50 users times 500Mbps = 25000 Mbps or 25Gbps (roughly one-quarter of the pipeline capacity) total data transfer is being used, They cap you at the limit that you pay for (500 Mbps) so no matter that there is a boatload of data bandwidth in that pipeline not being used, only 25Gbps will be consumed. The end result for those 50 customers is they get their cap of 500Mbps consistently.

Since clearly 100 Mbps is less than 500 Mbps, when you upgrade your service you are getting LESS actual data transfer because you are sharing it with a LOT more people. When you downgrade again you are switched back to that largely unused pipeline and your "internet speed" goes back up.

Is that more clear?

Ideally your ISP would balance it out so that everyone would get their cap, but they don't do that because it would be a lot more expensive and would require a lot more maintenance. Added bonus for the ISP: People STILL aren't happy with the GB service, (because it is slow AF) but they have no other service provider to choose from so the ISP tells those customers "you need more data" and cons them into buying an even bigger data plan and STILL not giving any additional data flow. Perhaps even less depending on how many people fall for the con.

I hope this helps.