Strange sorting

[Fred Bone]

In my bank-account register, most entries are blank in the "Num" column 
(e.g. for standing orders / direct debits, direct credits, ATM 
withdrawals). For cheques I record the cheque number (6 digits); for one-
off online transactions I often record the merchant's reference number 
(could be anything); etc.

Looking for a particular cheque number just now, I sorted the register on 
"Number".

As expected, entries with blank "Num" were first. However, the non-blank 
ones were not in the expected order, even after allowing for numeric 
filing after alphabetic. This makes finding the required entry something 
of a hit-and-miss affair. (Yes, I know I can use "Find" instead).

Among the all-numeric ones, I got:
001800 to 001886 (in order), then
000192
000193
001686 to 001699
001780 to 001799
003790
000163 to 000165
000172
001768 to 001769
001778 to 001779
002348
002378
002379
002618
002619
004558
004559
001700
001761 to 001767
001770 to 001777
002110
002345 to 002347
002350 to 002354
002376 to 002377
002616 to 002617
002620 to 002621
004410 to 004415
004555 to 004557
and so on.

Any comments?