Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3).
Why we need this RI in LTE concept? When UE experience bad SNR and it would be difficult (error prone) to decode transmitted downlink data it gives early warning to eNB by stating Rank Indication value as 1. When UE experience good SNR it pass this information to eNB by indicating rank value as 2.
Because of this reason, you might have observed that some time data transmitted by eNB is in Tx diversity mode, though MIMO was configured and hence you may have observed less downlink throughput than expected one.
However, it is not necessary that eNB will always change the transmission mode based on RI value, it could be implementation specific decision.
