The FULL values represent the measurements that were made on the TCH and the SACCH frames with a sample period of 20ms, assuming that every 20ms a decodable TDMA frame is received.
This means that, if DTX was active in a specific measurement period
(silence), the FULL values or RXLEV and RXQUAL may indicate bad.
The SUB values represent the measurements that were made on
the SACCH frames, which are repeated every 480 ms and which
carry the SYSTEM INFORMATION TYPE 5 and 6 (downlink) and the MEASUREMENT REPORTs (uplink). As a general rule, the SUB values always mirror the real radio conditions, but have a lower statistic reliability than the FULL values that were measured during speech periods.
It is definition and if you used DTX than only SUB values are OK.