We have 76k customers on shopify but on Klaviyo it only shows 33k active profiles

Hi,
For this client of mine, they have 76k customers on Shopify (customers who have given email consent), but on Klaviyo, I can only see 33k active profiles.
Can I know how this mismatch usually happens?
Also, let me know if, in this case, I can export the 76k customers from Shopify and upload them into Klaviyo.
And if I do, can I know how to do it properly?
Any help would be appreciated! Thanks
