They may do this for many reasons including:
1) They only have a low number of units and therefore give them low numbers like 10, 20, 30.
2) They are a big company like CSX (your picture) and they use various number-series to represent models. Let's say for example, the GP40-2s are number in the 10s, and they order 20 units. Well they'll have units number 10-19 (zero inclusive), making all 20 units in that series.
3) They may use a particular series for specific task or job like all units used for MOW may be numbered in the 20s, where as all units number in the 4500 series are used as road freight units, and are assigned to a southern mountain region.
John