It has to do with how it is reading say Subway 1 vs Subway 01. Subway 01 is a much more unquie whereas 1 could be wildcarded to *1 or 1*. I run into this in a lot of my access coding and if your naming goes beyond 100 you will find that subway 01 will not be as unique as 001. As 01 can be carded to *01 or 01*. Just a heads up when using a naming convention that makes use of a space. Subway without the space would work a lot better as the caption will see the space as an "OR" statement.
For future you should use 1 word and generate the max amount of zeros which relate the total number of images (x10) which will be used.