Hi,
Without any details about your current configuration (checkout workflow, payment method, etc), it will be difficult to say.
Maybe some customer are clicking several times ; or go back to re-submit.. A lot of things could happen which will be depend on your configuration.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.