In the "Transformers" film series, continuity errors and inconsistencies can sometimes occur between movies. While the films aim to create a cohesive universe, occasional discrepancies in the storyline or character histories may arise due to various factors, such as different directors, writers, or evolving creative decisions throughout the franchise.
In the first "Transformers" film released in 2007, Bumblebee's voice was portrayed as radio transmissions and sound clips from various sources, including songs and movie quotes. He never used his "real" voice in that movie.
However, in "Transformers: The Last Knight" released in 2017, it was revealed that Bumblebee did indeed have a "real" voice, and it was lost during his time on Cybertron. This revelation was meant to add depth to Bumblebee's character and provide new narrative opportunities for the franchise.
The inconsistency you pointed out regarding Optimus Prime's statement might be one of those continuity errors that can arise when multiple films are produced over an extended period. These errors can sometimes be overlooked or rationalized as creative choices to enhance the storytelling.
In movie franchises with an expansive universe, maintaining perfect continuity throughout all installments can be challenging. While filmmakers and writers often strive for consistency, some inconsistencies may still occur. Fans generally understand and accept these discrepancies, enjoying the films for their entertainment value rather than strict adherence to continuity.
It's important to remember that movies are fictional works meant to entertain, and certain creative liberties are often taken to enhance the storytelling experience.