Ordering in a drop down list

In my WordPress site, I created a drop down field with values starting from 80 to 230. However, when viewing this list, it starts from 100, 110, 120 and the numbers 80 and 90 are at the end. How can I fix it and have that list starting from 80, 90, 100, 110 etc ? Code is here: https://exal.gr/product/%CE%B1%CE%BD%CE%BF%CE%B9%CE%B3%CE%BF%CE%BC%CE%B5%CE%BD%CE%BF-%CE%B4%CE%B9%CF%86%CF%85%CE%BB%CE%BB%CE%BF-%CE%B1%CE%BD%CE%BF%CE%B9%CE%B3%CE%BF%CE%BA%CE%BB%CE%B9%CE%BD%CE%BF%CE%BC%CE%B5%CE%BD%CE%BF-kmg/
Image with the problem is here:
https://exal.gr/wp-content/uploads/2017/05/dropdown.jpg

Thanks

Ordering in a drop down list