Thanks for replying. I figured out the problem, and I actually thought I had deleted this post.
It turns out that I had messed up in importing the data into MySQL, using the wrong row delimiter, which somehow caused it to import every other row, except for June 2015. I reran the import today, and was more careful to make sure I imported every row. There are some inconsistencies across months in the posted data, with dates being formatted YYYY-MM-DD in the beginning, and m/d/YYYY beginning Sep. 2014, so that required some tweaking, too.
Here's the query:
select date_format(startTime,'%Y-%m') Month ,
format(count(bikeId)/max(dayofmonth(startTime)),0) AvgDailyTrips
from vw_TripData
group by date_format(startTime,'%Y-%m')
order by date_format(startTime,'%Y-%m');
And here's the new data, which looks like I'd expect it to look:
Month AvgDailyTrips
2013-07 27,207
2013-08 32,321
2013-09 34,479
2013-10 33,475
2013-11 22,526
2013-12 14,321
2014-01 9,690
2014-02 16,053
2014-03 14,165
2014-04 22,359
2014-05 27,939
2014-06 31,229
2014-07 31,253
2014-08 31,080
2014-09 31,796
2014-10 26,733
2014-11 17,640
2014-12 12,873
2015-01 9,211
2015-02 7,033
2015-03 11,027
2015-04 21,746
2015-05 31,032
2015-06 31,374
2015-07 35,022
2015-08 38,034
2015-09 42,990