x1   = 256080
x2   = 258160
diff = 2080.00000000003

##</code><code>##

x1   = 2.56080            e5
x2   = 2.5816000000000003 e5  (error ignored when printed)
diff = 2.08000000000003   e3  (error visible when printed)

##</code><code>##

  DB<115> $x1 = 2.56080e5
 => 256080

  DB<116> $x2 = 2.5816000000000003e5 
 => 258160                 # within error margin, ignored in normal display

  DB<117> printf '%.11f', $x2               
258160.00000000003         # forced into visibility, hence $x2 NOT an integer

  DB<118> $x2-$x1
 => 2080.00000000003       # error can't be ignored any more