Ethernet is the most widely used network and many analytical models were developed to predict its capacity. Usually in these models assumptions like infinite population, no back-off algorithm and slotted channel are made. By comparing two models (one slotted and the other non-slotted) with reported measurements, we showed the optimistic estimate of the channel capacity of the slotted model is not due to slotting the channel as believed in the literature but due to other assumptions. Finally, the effect of difference between IEEE 802.3 and Ethernet DIX version 1 regarding preamble sending (IEEE 802.3 completes sending the preamble and then sends the jam signal while Ethernet DIX version 1 aborts sending the preamble and sends the jam signal immediately) on degrading the channel capacity was analyzed.