Isn't the PCF8574 an I2C to 8-bit interface? The HD44780 display has a few control lines in addition to data, so you are probably running in 4-bit mode, with 4 of the pins for data plus RS, RW and EN. Each character will take two complete transactions. And the EN pin is how you clock in each byte. If you have the EN pin connected to your PCF8574 you will need four I2C instructions to print one character. If you clock the display separately from the PCF8574 then it only takes two I2C writes, but they need to be interspersed with clocking of the display's EN pin.