Powershell giving wrong output in if else block

Check the following code with simple if else blocks

cls
$a = Read-Host("Enter the marks ")

if (($a -le 100) -and ($a -ge 90)){

 Write-Host("The grade is A")

}
elseif (($a -le 89) -and ($a -ge 80)){

 Write-Host("The grade is B")

}

elseif (($a -le 79) -and ($a -ge 70)){

 Write-Host("The grade is C")

}

elseif (($a -le 69) -and ($a -ge 60)){

 Write-Host("The grade is D")

}
elseif($a -lt 60){
Write-Host("The grade is F")
}

And when I give input between 90 - 99 it returns without any output. And if I give 100 as input, I am getting "The grade is F", ie, the else block is executed, but it should not work according to the code. Can anyone explain why?