Hi,
Because you wanted sorting by ‘_sku’ ascending, and the other matches have alphabetically preceeding SKUs, therefore it preceeds the relevance sorting.
If you want more ‘_sku’ relevant results first, you should try to change the ordering back to default: http://i.imgur.com/qrm0rwX.png
I have configured and enabled the index table engine for you, and also made the custom field (sku) relevance higher. If you change the ordering to relevance, then SKU related matches will start appearing closer to the top.