int(lambda*exp(-lambda*t), t=0..infinity) 

will get the integration result of 1

if we replace lambda with real numerical value of 1e5

we will get the integration result of 1

if we replace lambda with real numerical value  of1e6

we will get 0

but if we set lambda with an integer

lambda:= 1000000

the integration would be correct and equal to 1

why is this ?