Why my order increment ID is not continuous

[dosenje]

I used magento 2.1.5 to build a website.

I found that my order ID is not continuous, there will be an indefinite number of cross.

For example, the current order from the ID is 0000052, the next one is 0000062. Why is this?

I see through the phpmyadmin data table found, sequence_order_1 in the data will be a lot of useless.

My question is why the table(sequence_order_1) will be so abnormal.

thanks

[Tom Robertshaw]

Order IDs can be reserved against quotes but then not used if that basket didn't get all the way to an order. I think that could be why this happens. 

[Sheba]

Hi Tom,

 I am using Magento 2.2.1.  Which files keeps the order numbers? Thanks for your attention.