title: scikit-learn roc_curve: why does it return a threshold value = 2 some time?
body: <p>Correct me if I'm wrong: the "thresholds" returned by scikit-learn's roc_curve should be an array of numbers that are in [0,1]. However, it sometimes gives me an array with the first number close to "2". Is it a bug or I did sth wrong?  Thanks.</p>
  
  <pre><code>In [1]: import numpy as np
  
  In [2]: from sklearn.metrics import roc_curve
  
  In [3]: np.random.seed(11)
  
  In [4]: aa = np.random.choice([True, False],100)
  
  In [5]: bb = np.random.uniform(0,1,100)
  
  In [6]: fpr,tpr,thresholds = roc_curve(aa,bb)
  
  In [7]: thresholds
  Out[7]: 
  array([ 1.97396826,  0.97396826,  0.9711752 ,  0.95996265,  0.95744405,
      0.94983331,  0.93290463,  0.93241372,  0.93214862,  0.93076592,
      0.92960511,  0.92245024,  0.91179548,  0.91112166,  0.87529458,
      0.84493853,  0.84068543,  0.83303741,  0.82565223,  0.81096657,
      0.80656679,  0.79387241,  0.77054807,  0.76763223,  0.7644911 ,
      0.75964947,  0.73995152,  0.73825262,  0.73466772,  0.73421299,
      0.73282534,  0.72391126,  0.71296292,  0.70930102,  0.70116428,
      0.69606617,  0.65869235,  0.65670881,  0.65261474,  0.6487222 ,
      0.64805644,  0.64221486,  0.62699782,  0.62522484,  0.62283401,
      0.61601839,  0.611632  ,  0.59548669,  0.57555854,  0.56828967,
      0.55652111,  0.55063947,  0.53885029,  0.53369398,  0.52157349,
      0.51900774,  0.50547317,  0.49749635,  0.493913  ,  0.46154029,
      0.45275916,  0.44777116,  0.43822067,  0.43795921,  0.43624093,
      0.42039077,  0.41866343,  0.41550367,  0.40032843,  0.36761763,
      0.36642721,  0.36567017,  0.36148354,  0.35843793,  0.34371331,
      0.33436415,  0.33408289,  0.33387442,  0.31887024,  0.31818719,
      0.31367915,  0.30216469,  0.30097917,  0.29995201,  0.28604467,
      0.26930354,  0.2383461 ,  0.22803687,  0.21800338,  0.19301808,
      0.16902881,  0.1688173 ,  0.14491946,  0.13648451,  0.12704826,
      0.09141459,  0.08569481,  0.07500199,  0.06288762,  0.02073298,
      0.01934336])
  </code></pre>