TL;DR version:
This isn't really a bug. It just has to do with inaccurate descriptions of reached and exceeded. Here are better descriptions:
exceeded = earned more than 200 net reputation in one day
reached = earned 200 or more rep in one day purely from received votes (up or down)
EDIT: per Nick Craver's edit to the list of badges (see revision 25), there's yet another definition exception to consider: the +100 account association bonus does not count towards "exceeded."
The contents of this answer were previously posted as edits to the question.
Based on a suggestion by badp, I checked my audit history for every day I hit the cap. On two of those days, I offered bounties. So, even though I hit the cap, my total rep change ended up at +106 in one case (100-pt bounty) and -285 in the other (500-pt bounty). If this is the cause, then the definition for "exceeded" should perhaps be amended to be:
earned more than 200 net reputation in one day
But wait! There's more! My March 11, 2011 rep looks like this:
Vote rep | Grand total | Upvotes only
----------+-------------+--------------
(10) | 10 | 10
(10) | 20 | 20
(10) | 30 | 30
(-1) | 29 | 30
(10) | 39 | 40
(10) | 49 | 50
(10) | 59 | 60
(10) | 69 | 70
(10) | 79 | 80
(10) | 89 | 90
(-2) | 87 | 90
(10) | 97 | 100
(10) | 107 | 110
(10) | 117 | 120
(-2) | 115 | 120
(10) | 125 | 130
(10) | 135 | 140
(10) | 145 | 150
(-2) | 143 | 150
(10) | 153 | 160
(-1) | 152 | 160
(10) | 162 | 170
(10) | 172 | 180
(-2) | 170 | 180
(-2) | 168 | 180
(-2) | 166 | 180
(-2) | 164 | 180
(-2) | 162 | 180
(10) | 172 | 190
(-2) | 170 | 190
(10) | 180 | 200 Should be "reached" on this line,
(-2) | 178 | 200 if the system
(-2) | 176 | 200 really does care
(10) | 186 | 210 about only upvotes,
(10) | 196 | 220 but rep actually
(-2) | 194 | 220 keeps on
(-2) | 192 | 220 counting until...
[8] | 200 | 230 this line
[0] | 200 | 240
[0] | 200 | 250
...
This proves that just looking at upvotes isn't enough for determining "reached." It also suggests that downvotes cast are irrelevant. So it looks like the definition for "reached" should perhaps be amended to be:
earned 200 or more rep in one day purely from received votes (up or down)