Whilst the server's ISP plan might be 100 Mbit/s, the actual speed will always be less. There is always speed loss due to various reasons. One is loss over line distance between ISP's, another is loss due to equipment used such as routers & even the server computers themselves. Average speed loss is generally between 15-25% overall depending on those & perhaps other factors. Also uploading is technically more difficult than downloading. So you would not expect the entire full bandwidth available to be used up solely for uploading. The ISP may also have some type of uploading limitations, perhaps even due to their equipment.

But yes I agree, something does not seem right that you are only getting such speeds from that server. My only guess is that the LW on your server is using up much of its upload bandwidth for other people downloading from it. Any chance of getting someone to check the upload window on the server's LW('s)?