If you hover over a subscriber count on the website, it will give you the exact count.
The difference you are talking about is caused by the app rounding up, example:
Somebody has 1451 subscribers, the site will show it as 1.4k, while the app rounds up to 1.5k.
At 1449 subscribers, both app and website will show 1.4k.
I don't know why they selected to do this, it may just be an oversight.