The data in the resultset is obtained after executing the query and not from the table.
In the resultset (join of two tables), UPDATE query is fired when you try to modify the value of an existing row. INSERT query is fired in case of a new row is added.
I think SQLyog should be smart enough to understand that skuclassificationskuid is the primary key of the sku_classification_sku table, so an update with a null PK doesn’t make sense, so an insert is needed instead.