Remember nothing is erratic there. Based on your observation above your going by a sort not the prev next links which are simply higher or lower product number links
Yes clicking the first product and doing next goes to the 5th product cause of the product numbers 1698 and 1702. It is not going to go to 1706
Previous would go to a lower product and next would go to a higher product. My showcat sort is set to most active. If you change the sort to most recent then you see your prev or next may line up more with your thinking
Therefore based on your observation and my explanation I conclude they are working as they are designed.