Spotted this on reddit https://www.reddit.com/r/openstreetmap/comments/1hd5coe/why_would_they_not_label_interstate_numbers_on/ - same as other note, this is not a data problem but a render issue.  For the most part the standard/carto is what's used as baseline for data inspection at a glance, but pulling a map extract from the database is final say whether a road is labeled or not.