Complex sentence: They were built in 1891 and 1892 and were numbered in the range from 15 to 20, but for some reason, possibly their delivery sequence, their numbering was not in the same sequence as their works numbers.
Split into simpler sentences: They were built in 1891 and 1892 and were numbered in the range from 15 to 20. For some reason, possibly their delivery sequence, their numbering was not in the same sequence as their works numbers.