Dear Nasser,
about the rank, this is correct. Only for very good channel conditions, there will be an improvement when using two streams in a 2×2 setup.
Why you see the increase in throughput comes from a higher SINR when using the transmit and receive antennas for diversity.
You can also check this by looking at the average CQI. It will be significantly higher for a 2×2 setup than for a 1×1 setup.
Best regards,
Martin