Unseasonably cool weather failed to dampen May sales at Wal-Mart Stores Inc, which yesterday posted a 6.2 per cent increase in same-store sales - ahead of analysts' expectations and up from 3.3 per cent the month before. For May, the nation's largest retailer said same-store sales at its Wal-Mart division rose 6.8 per cent, and increased 3.7 per cent at Sam's Club.