Leaving aside that the 32GB phone is reported to have a read speed *double* that of the higher-capacity phone, it's my understanding that flash drives typically achieve faster write speeds by writing to multiple flash modules in parallel, since individual flash module write speeds are so slow compared to their read speeds. This can be seen in the read/write speed specs by drive size within many SSD lines.
If a single flash module in these phones is 32GB (as seems likely), then the 256GB phone model would have 8 such modules, resulting in 8x parallelism on write, and a predicted write speed 8x that of the 32GB model. Fits the data, doesn't it? No resort to a "cheap flash" conspiracy needed. :)