CRSP has two types of cap based deciles.
1) CRSP Stock File Capitalization Decile Indices: Largest companies in decile 10.
2) CRSP Cap-Based Portfolios: Largest companies in decile 1 (not 10).
The two methodologies were invented twenty years apart. They contain slightly different universes of stocks. CRSP / WRDS do not make a recommendation on which methodology to use.