And there is a difference between יום (day) and בקר (morning): The noun יום is said about all of the day, while the noun בקר is said about the time of waking up early in the morning.

And בין הערבים (dusk) is sometimes from the sixth hour on and sometimes close to evening. And there is a difference between ערב (evening) and בין הערבים (dusk): The noun ערב when it comes versus בוקר divides the daytime into two halves, but the noun ערב when it comes versus the noun יום is literally night. And with this it needs to say בין הערבים to indicate that it is in the afternoon. But when the noun בין הערבים comes versus בוקר, it is close to darkness, since the afternoon is indicated with the word ערב.