You'd have to ask Asda. But it's not Asda that accepts or doesn't accept more than £30, it's each individual card reader. "Stupid" card readers only see a conctactless card and have the £30 limit. "Clever" card readers see it is Apple Pay and accept higher limits. It could easily be that some stores have upgraded their card readers and others haven't.
Asda is owned by Wal-Mart. To date, Wal-Mart has never liked contactless payments as it hinders their data collection and mining efforts. They have not enabled it in US or Canadian Wal-Mart stores (even though Canada has a contactless use and acceptance rate nearing that of the UK).
The only reason you see it being enabled at all is to satisfy mandates from the card networks that all card readers in Europe need to support contactless. Unfortunately, this mandate does not include requirements to support Device Verification for transactions exceeding the No Verification amount (£30 in the UK).