A puzzling arithmetic error in Maxima

Hi:

 This same result can be seen in Scilab also:

-->//Set display to 18 digits                                                          
-->format(18)                                                                          
-->57.50+14.95+18.95+8.95+22.95+12.95+7.95+6.95+19.95+12.95+8.95+9.95+16.95
+19.95+11.95
 ans  =
 
    251.7999999999999  
 
-->// Happens here in Scilab too