Internet Bandwidth

My company’s network had 128K leased line connection to our ISP. Browsing Internet was very slow sometimes, so we upgraded to 512K.

After monitoring the bandwidth usage over one month, it is found that maximum bandwidth usage is varying between 60k and 90k(this test was not done, when the connection was 128K)

Even though more than 30 users are browsing Simultaneously; bandwidth is not crossing 90K, even though browsing speed is reasonably good.

Can somebody tell me why the maximum bandwidth is not used? Why it is not even reaching the 1/4 of the bandwidth?

Thanks.