Rack enclosure dimensions
Solution 1:
Cable management arms and cables themselves need room too. Also, not all servers and devices are the same depth, so that explains the differences in enclosure depths.
Solution 2:
Because you need space for cables, etc.
Also your specific servers may be 600mm deep but Dell servers in general come in a variety of depths.
Solution 3:
What if you move to blades later, their enclosures are longer than 600mm usually.
Solution 4:
The extra space in back of the rack can be very nice for mounting "zero-U" PDUs in addition to giving you plenty of space for cable management. I've worked with many racks that were horribly under-sized for their contents, and it's not fun. I'd highly suggest you go with the 800mm at the least, if not the 1000mm..