Comparing variables in arithmetic

Was wondering if someone can take the time to explain the following:

I have a directory of files (PDF), which I place into an array.

shopt -s nullglob                                   # Set array to 0 is nothing found=
declare -a TotalFiles=($Prefix*.pdf)                # Current listing of files
TotalFileCount=${#TotalFiles[@]}

In my mesting the array contains the following.

Array Contents Scan-0030.pdf Scan-0140.pdf Scan-0005.pdf Scan-0006.pdf Scan-0007.pdf Scan-0008.pdf Scan-0009.pdf Scan-0010.pdf

I have created a the following functions to derive the next file to create

function NextNum {
    HighestNum =0
    echo "NextNumber Functions"

    #for index in "${TotalFiles[*]}"
    for file in ${!TotalFiles[*]}
    do
        #printf "%4d: %s\n" $index $TotalFiles ${array[$index]}
        echo $file ${TotalFiles[$file]}

        name=${TotalFiles[$file]}
        name=${name//[^0-9]/}
        name=$((10#$name))

        echo "File number in  name  - $name"
        echo $file
        TotalFiles[$file]=$name

        **((name > HighestNum)) && HighestNum=$name**
    done
}

My question is with this line in the function which i found by googleling.

((name > HighestNum)) && HighestNum=$name

How come one does not have to specify that two variables are being compared? like this,

(($name > $HighestNum)) && HighestNum=$name

thank you for the assistance.