Focus: bringing attention to the present situation or topic
Concentration: maintaining attention on the present situation or topic
'Getting present' is a Focus exercise...a mind-process of narrowing and fixing attention.
'Staying present' is a Concentration exercise...a mind-process of holding attention.
These two mind-processes, focusing and concentrating, involve different areas of the brain and they require the development of different skill sets.
For me, and it may be different for you, focusing is an exercise of thought discipline. It requires thought discipline to cause attention to dwell on the single isolated situation or topic, ignoring everything else. This discipline can be very challenging. Similarly, concentration can be challenging when tasks are tedious or uninteresting. On the other hand, I find concentration easy when the topic or situation is one I am very interested in or excited about.