An alternate pronunciation of 'Derek' is 'DER-ək', using a schwa instead of in /i/. It sounds almost like an /e/, although there's a slight difference. It's easier to pronounce the name with a schwa (like above) than it is to pronounce it 'DER-ik', because of how the tongue has to move to make the sounds. A lot of the time you automatically choose the easier way to save time/energy/because it's easier etc. - that's why some people say 'fink' instead of 'think'...