It’s just the way CP/WP and CC/WC work. Orders in CC/WC are just custom post types and are given the next available post ID as any other post would.
In a typical scenario, Customer A creates an order. The order is given order number #1234 which happens to be the next available post ID. The website owner then adds a new product which is assigned post ID #1235 and a product image which gets post ID #1236. They then create a blog post about the new product and this is given ID #1237.
Then Customer B comes along and places a new order. The new order number is…? You guessed it: #1238.
There are one or two premium plugins designed to work around this BUT, of the two that I’ve found, their minimum requirements are such that there is no guarantee that either will work on CP/CC.
The plugins are:
Sequential Order Numbers Pro (by SkyVerge available from woocommerce.com)
Custom Order Numbers for WooCommerce (by Tyche Software)
The latter is only 18 USD at present as they’ve got a Black Friday sale on if you want to give it a try.