Because streaks and focus statistics are recorded per device and do not sync. Your iPhone counts the days you studied on your iPhone. Your iPad counts its own. Neither number is wrong, and neither is the total.
The same applies to focus sessions today, minutes focused today, and the seven-day review chart. All of them are local to the device you are looking at.
Everything that matters to your learning. Classes, subjects, cards, their photos and drawings, every card's review schedule, and your deadlines with their prep bars. Those move between devices through your own iCloud account and stay consistent.
The habit numbers are there to tell you something about your week, and they do that job on whichever device you actually study on. Merging partial counts across devices would produce a number that is harder to interpret rather than easier, and it would mean sending more data through sync for a figure that changes nothing about what you review.
Most people have one device where the real work happens, usually an iPad or a Mac at a desk, with a phone used in queues and waiting rooms. Read the streak on the device you study on and treat the others as incidental.
More answers on the help page, or write to [email protected].