Percentages taken affected by sampling interval

[agrimm]

As I decrease the sampling interval, starting at 1000 microseconds and going down to 10 microseconds, when sampling by wall time, the proportions used by at least one method decreases:

[redacted]#run_clustalw_command [redacted]
  samples:  6051 self (55.2%)  /   6051 total (55.2%)
  callers:
    6051  (  100.0%)  [redacted]#[redacted]
  code:
                                  |   143  |   def run_clustalw_command(clustalw_command)
                                  |   144  |     # [redacted]
 6051   (55.2%) /  6051  (55.2%)  |   145  |     system(clustalw_command) or fail "clustalw failure"
                                  |   146  |   end

[redacted]#run_clustalw_command [redacted]
  samples:  11982 self (54.0%)  /   11982 total (54.0%)
  callers:
    11982  (  100.0%)  [redacted]#[redacted]
  code:
                                  |   143  |   def run_clustalw_command(clustalw_command)
                                  |   144  |     # [redacted]
 11982   (54.0%) /  11982  (54.0%)  |   145  |     system(clustalw_command) or fail "clustalw failure"
                                  |   146  |   end

[redacted]#run_clustalw_command [redacted]
  samples:  16123 self (21.0%)  /   16123 total (21.0%)
  callers:
    16123  (  100.0%)  [redacted]#[redacted]
  code:
                                  |   143  |   def run_clustalw_command(clustalw_command)
                                  |   144  |     # [redacted]
 16123   (21.0%) /  16123  (21.0%)  |   145  |     system(clustalw_command) or fail "clustalw failure"
                                  |   146  |   end

[redacted]#run_clustalw_command [redacted]
  samples:  18807 self (10.4%)  /   18807 total (10.4%)
  callers:
    18807  (  100.0%)  [redacted]#[redacted]
  code:
                                  |   143  |   def run_clustalw_command(clustalw_command)
                                  |   144  |     # [redacted]
 18807   (10.4%) /  18807  (10.4%)  |   145  |     system(clustalw_command) or fail "clustalw failure"
                                  |   146  |   end

Is this a normal occurrence, or a potential bug?