title: 'predict' gives different results than using manually the coefficients from 'summary'
body: <p>Let me state my confusion with the help of an example,</p>
  
  <pre><code>#making datasets
  x1&lt;-iris[,1]
  x2&lt;-iris[,2]
  x3&lt;-iris[,3]
  x4&lt;-iris[,4]
  dat&lt;-data.frame(x1,x2,x3)
  dat2&lt;-dat[1:120,]
  dat3&lt;-dat[121:150,]
  
  #Using a linear model to fit x4 using x1, x2 and x3 where training set is first 120 obs.
  model&lt;-lm(x4[1:120]~x1[1:120]+x2[1:120]+x3[1:120])
  
  #Usig the coefficients' value from summary(model), prediction is done for next 30 obs.
  -.17947-.18538*x1[121:150]+.18243*x2[121:150]+.49998*x3[121:150]
  
  #Same prediction is done using the function "predict"
  predict(model,dat3)
  </code></pre>
  
  <p>My confusion is: the two outcomes of predicting the last 30 values differ, may be to a little extent, but they do differ. Whys is it so? should not they be exactly same?</p>