GPON (the technology behind UFB Bitstream 2) is asymmetric. 2.5Gbps downstream, 1.25Gbps upstream (before overheads). That bandwidth is shared across a number of customers, usually 16 with the current split.

If you only have 1.26 Gbps of upstream bandwidth, you really don't want to be offering 1Gbps of upstream shared over 16 customers, at least not if you want them to actually be able to get it, most of the time. A general rule of thumb says you don't want to offer more than about half the trunk bandwidth (25% would be safer).

That's the base reason behind asymmetric speed offerings on UFB 2 (and 3)—that's what the underlying technology delivers.