I'm not in marketing but it totally makes sense from a marketing standpoint.
Largest option: Like a lot of people, I always get the largest capacity available just because you can never have too much storage since you cannot add afterwards and yes, I do use a lot of my storage (and would not want to consistently have to get rid of stuff like I do on my MacBook Air with the largest storage available at the time). I currently have the 128gb and am just at 97.7gb so obviously the middle option 64gb would have been too small for me.
Middle option: BUT, the current 64 *is* enough for the majority of iPhone users who do keep a normal amount of photos, videos, music, etc., on their personal phone.
Basic option: Then of course, there are the gazillion businesses that buy iPhones for corporate-only use -- and those would most likely be basic models with enough storage for some media for work purposes (depending on the industry, of course) with no need for space for movies, music, etc. There are also some people who still actually use their phone just as a phone. For example, my older relatives have iPhones just for emergencies. So, the first tier phone (currently 16gb) is fine for this group.
For this 7 series, of course storage capacity should get bumped up a notch. And I'm guessing the idea behind the marketing: they offer the basic option (32), middle (128) and top (256) because if they start at 64 as the basic model, everyone in groups one and two will all buy the lowest-priced basic option and the basic group just scored a whole lot of unnecessary extra storage. Apple would rather have that extra $100 or so dollars from the majority of users who would be fine with 64gb but now they are getting double the storage so they're counting on an easier decision on the buyer's part to choose the bigger capacity and pay the extra money.
That said, I am really hoping they do the 256! It will be tougher to back up my phone to my MacBook Air, but I'll just have to get a bigger one soon!