Hi,
The variant with the combination "Clear / M" does not exist.
At this moment HikaShop can't select it as the default value because the value does not exist.
So you have ot choose a default combination which exists or, create the variant for the combination "Clear / M".
And for the question "Why the variant 'Athletic Heather XS' is selected ?"
It's because it is the first variant in your listing (with the lower position or at least, the lower product_id).
The default combination does not exist, so HikaShop has to add something in the cart, it take the first variant it found for the variant when it sees that the default does not exist (instead of displaying you a big error message which will afraid the customer)
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.