Category surrogate keys are added on product pages. Due to this all cached product pages are flushed when a category needs to be flushed.
By removing the surrogate key from the product page, we can avoid this making overall cache more effective.
Here is that it looks like on a typical product page.
Surrogate-Key: cat_c c9 c18 c4 c201 c6 c206 c208 store cb cb_weltpixel_pre-footer cb_weltpixel_footer_v3 p103697 cat_p cb_weltpixel_global_promo_message_2 text
All the c9 c18 surrogate keys are coming from the categories populated in Top Navigation. So when C18 is invalidated the all pages of magento are technically flushed.