Ethernet network speed

We have a 10/100 MB/s autosensing hub LAN running Ethernet peer to peer. Computers on the network are Pentium 133 MHz or higher with IDE disk drives and running Win ?98. The hub indicates that communication is occuring at 100 MB/s. The collisions indicator rarely flashes and when it does the 100 MB/s reverts to 10 MB/s momentarily. In timing the transfer of either one large file or a group of files from computer to computer via Window Explorer, I noticed that it takes approximately 14 minutes to transfer 1.5 GB. If each byte is 8 bits then there are 12 Gigabits or 12,000 Megabits. At 100 MB/s this would result in a transfer time of 120 seconds. Why is the transfer slower than expected?