Q: Image file name sequence inconsistent with the order of date taken

With iphone5c icloud "backups", I noticed where the image file name sequence (e.g., IMG_0001, IMG_0002, IMG_0003) is inconsistent with the order of date taken.  How would this happen?  Is it even possible?

For example:

Image Name          Date/Time taken

IMG_0001               08/20/2016   13:30:45

IMG_0002               08/20/2016   13:35:20

IMG_0003               08/20/2016   13:32:34

IMG_0004               08/20/2016   13:39:25

IMG_0007               08/20/2016   13:20:15

IMG_0011               08/20/2016   13:45:22

IMG_0012               08/20/2016   13:52:33

IMG_0018               08/20/2016   13:51:10

Thanks in advance,

Jv