The RLC payload size is fixed to 40 bytes in Release 99 for Acknowledged Mode Data.
An additional configuration option to use an 80-byte RLC packet size was introduced in Release 5 to avoid extensive RLC protocol overhead, L2 processing and RLC transmission window stalling.
Sinc HSDPA+/HSUPA+ allow the optimization of the L2 operation since L1 retransmissions are used and the probability of L2 retransmissions is very low. So it is possible to use flexible and considerably larger RLC sizes and introduce segmentation to the Medium Access Control (MAC) layer in the base station.
I hope this should answer your second part of the question, now coming to first part why you think that datarate should be different, the purpose of flexible PDU was different (adding flexibility to avoid various overheads).
Check this article for more clarification