stats plugin: discrepancy between request histogram and total

The per-request-size counts do not agree exactly with the reported totals (even with statsthreshold=100). For example:

total: 820 ops, 54.551980 s, 12.29 MiB, 230.61 KiB/s
read: 162 ops, 0.000155 s, 4.41 MiB, 27.76 GiB/s op, 82.71 KiB/s total
write: 631 ops, 0.003885 s, 7.88 MiB, 1.98 GiB/s op, 147.90 KiB/s total
flush: 27 ops, 0.000008 s, 0 bytes, 0 bytes/s op, 0 bytes/s total

READ request sizes:
     4096 bytes: 59.3% of requests (96)
         12 bit aligned: 100.0% (96)
         13 bit aligned:  69.8% (67)
         14 bit aligned:  47.9% (46)
         15 bit aligned:  38.5% (37)
         16 bit aligned:  32.3% (31)
         17 bit aligned:  26.0% (25)
         18 bit aligned:  19.8% (19)
         19 bit aligned:  16.7% (16)
         20 bit aligned:  13.5% (13)
         21 bit aligned:  10.4% (10)
         22 bit aligned:   7.3% (7)
   114688 bytes:  7.4% of requests (12)
         14 bit aligned: 100.0% (12)
     8192 bytes:  5.6% of requests (9)
         12 bit aligned: 100.0% (9)
         13 bit aligned:  33.3% (3)
    16384 bytes:  3.7% of requests (6)
         13 bit aligned: 100.0% (6)
         19 bit aligned:  50.0% (3)
    28672 bytes:  1.9% of requests (3)
         12 bit aligned: 100.0% (3)

WRITE request sizes:
     4096 bytes: 67.8% of requests (428)
         12 bit aligned: 100.0% (428)
         13 bit aligned:  48.6% (208)
         14 bit aligned:  27.3% (117)
         15 bit aligned:  16.8% (72)
         16 bit aligned:   9.8% (42)
         17 bit aligned:   4.9% (21)
         18 bit aligned:   3.3% (14)
         22 bit aligned:   2.1% (9)
     8192 bytes: 11.4% of requests (72)
         12 bit aligned: 100.0% (72)
         13 bit aligned:  43.1% (31)
         14 bit aligned:  18.1% (13)
         15 bit aligned:  12.5% (9)
         16 bit aligned:   9.7% (7)
         17 bit aligned:   4.2% (3)
         18 bit aligned:   1.4% (1)
    12288 bytes:  4.9% of requests (31)
         12 bit aligned: 100.0% (31)
         13 bit aligned:  48.4% (15)
         14 bit aligned:  22.6% (7)
         15 bit aligned:  16.1% (5)
    16384 bytes:  4.6% of requests (29)
         12 bit aligned: 100.0% (29)
         13 bit aligned:  31.0% (9)
         14 bit aligned:  10.3% (3)
    20480 bytes:  1.9% of requests (12)
         12 bit aligned: 100.0% (12)
         13 bit aligned:  75.0% (9)
         14 bit aligned:  41.7% (5)
         15 bit aligned:  33.3% (4)
   114688 bytes:  1.7% of requests (11)
         12 bit aligned: 100.0% (11)
         14 bit aligned:  90.9% (10)
    32768 bytes:  0.8% of requests (5)
         12 bit aligned: 100.0% (5)
         14 bit aligned:  40.0% (2)
         22 bit aligned:  20.0% (1)
    53248 bytes:  0.6% of requests (4)
         13 bit aligned: 100.0% (4)
         22 bit aligned:  50.0% (2)
   253952 bytes:  0.5% of requests (3)
         13 bit aligned: 100.0% (3)
    45056 bytes:  0.3% of requests (2)
         12 bit aligned: 100.0% (2)
         17 bit aligned:  50.0% (1)
    77824 bytes:  0.2% of requests (1)
         22 bit aligned: 100.0% (1)

TRIM request sizes:
    (no such requests)

ZERO request sizes:
    (no such requests)

Summing up the writes, we get 428+72+31+29+12+11+5+4+3+2+1 = 598 - not 631.

For the reads we get 96+12+9+6+3 = 126 - not 162.