Percentage calculation glitch

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.3

Got a strange glitch today
Everything was fine but today someone made an order and use coupon for 5%

Subtotal $11.90
Coupon $-0.60 FROMEBAY
Shipping $0.00- NONE
Payment fee $0.00 - PayPal
Total $11.31

Why $11.31 not 11.30?
After he pushed button Finish next page was PayPal page with $11.30 but since 11.30 =/= 11.31 order cancelled

I found why
Send details of the order i checked it today
After i unchecked box everything ok now