Main order number jumps from 3401 to 3404

-- HikaShop version -- : 2.6.4
-- HikaMarket version -- : 1.7.2
-- Joomla version -- : 3.6.4
-- PHP version -- : 7

Use same order number for sub-orders is set to: YES

This is just an example:
When a customer creates an order (consisting of items from example 2 Vendors)
Then. i.e the main order get "order number 3401"

...then the next customer creates and order, and his main order gets "order number 3404"

Why is this the case ?
Why doesn't the last order get order number 3402 (So it follows normal order) ?