It would be unusual for your ISP’s count to exactly match your count because they will be measuring throughput slightly differently than GlassWire. But a 20% difference is unlikely to explain this.
If you want to narrow it down further then you will need to provide more information about your setup so the extensive list of possible explanations can be reduced to a shortlist. Remember that GlassWire only records throughput for devices it is installed on.
So what is your setup?
- What type of Internet connection, e.g. ADSL/VDSL, cable, optical fibre, wireless?
- Do you use any gateway (to connect to the Internet) devices such as modems and routers?
- What devices use the Internet connection, e.g. PCs, mobile phones?
- How is each device connected wireless or wired or both?
- How do you use your connection including do you use VPNs, proxy servers, torrents, etc?
By the way, the usual recommendation is to rely on what your ISP tells you because they don’t take any notice of anything else, e.g. https://www.howtogeek.com/192654/how-to-monitor-your-internet-bandwidth-usage-and-avoid-exceeding-data-caps/
Your ISP’s tool is the best way to stay up-to-date on how much data you’re using. No matter how well you monitor your own data, your ISP will always use their own numbers to decide how much data you’ve uploaded and downloaded.
The downside of your ISP’s tool is that it may not be updated very frequently. For example, some ISP’s may update this bandwidth usage meter every day, although some may update it more frequently. Tools you use yourself can give you up-to-the-minute bandwidth usage information.
So use GlassWire to indicate your usage and check in with your ISP as you get closer to the end of the billing period.
P.S. I forgot to say that I linked to the article above because it provides instructions for other methods to check your data usage if you have Windows 8/8.1/10 or your router.