Why there are 2 group A and B in preamble in LTE?

Hi Experts.
Why there are 2 group A and B in preamble in LTE?

If group A is received, smaller UL grant is provided in RAR.
If group B is received, larger UL grant is provided in RAR.

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Group A using when UE transmit short message ,this selection indicate radio channel quality is poor
Group B using when UE transmit large message, this selection indicate radio channel quality is good

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