That's by design. Virtual disks on an ESXi host always consist of 2 files, a small header/descriptor file, and the data (flat, delta, or sesparse) .vmdk file. Likely to avoid confusion (just my personal guess), VMware only shows the header file's name with the data file's size.
Data file names (flat/delta/sesparse) only show up in the GUI if there's an issue with the corresponding header file (corrupt, or missing).
André