Guessing but maybe you can read and write at the same time, so actually you should be multiplying by 3 not 4. Although this would give you 90gb/sec (which i though is moreless that what generally cuda cards achieve)
Yeah, reaching theoretical bandwidth is rarely, if ever, possible. From what I’ve gathered, 2/3 theoretical is about normal. I get around 48 GB/s. These differences can come from MoBo and driver issues, and other hardware issues. I presume there are other reasons as well, but these are common.