For some reason, Sandisk's 64 GB CFast card only has half the write speed it should due to the way they engineered the 64 GB variant. It only has a write-speed of 240 MB/s where as Sandisk's 128 GB CFast card has a write speed of 440 MB/s.
Solid state storage devices (including SSDs) consist of multiple flash chips. Although a vendor will likely have a variety of chip capacities available, ultimately most models will consist of more than one actual flash part to make up the total amount of storage.
Each chip will have a maximum read/write speed, so for a larger capacity variant of the product, you are likely to find there will be more flash chips, and the internal controller can write data to multiple chips in parallel.
If you look at the early days of SSDs the smaller capacity devices were usually slower, up until the midrange capacity where the performance was high enough to saturate the SATA bus, and then higher capacity models (whilst possibly able to internally write/read faster) were then no quicker as they were limited by the bus.
My guess is that the Sandisk 64GB model may be using fewer (larger) flash chips vs the Lexar. The Sandisk 128GB model may well have the same sized chips as their 64GB variant, but obviously more. I could be wrong, and Lexar might just have faster flash chips.
Writing with flash is usually harder/slower than reading, hence the disparity between the read and write speeds.