Implementation of calculate ip checksum is different from my original exapmle

I want to implement IP header checksum and fount this:

public static ushort InternetChecksum(this IEnumerable<byte> value)
{
    byte[] buffer = value.ToArray();
    int length = buffer.Length;
    int i = 0;
    UInt32 sum = 0;
    UInt32 data = 0;
    while (length > 1)
    {
        data = 0;
        data = (UInt32)(
        ((UInt32)(buffer[i]) << 8)
        |
        ((UInt32)(buffer[i + 1]) & 0xFF)
        );

        sum += data;
        if ((sum & 0xFFFF0000) > 0)
        {
            sum = sum & 0xFFFF;
            sum += 1;
        }

        i += 2;
        length -= 2;
    }

    if (length > 0)
    {
        sum += (UInt32)(buffer[i] << 8);
        //sum += (UInt32)(buffer[i]);
        if ((sum & 0xFFFF0000) > 0)
        {
            sum = sum & 0xFFFF;
            sum += 1;
        }
    }
    sum = ~sum;
    sum = sum & 0xFFFF;
    return (UInt16)sum;
}

So i took this implementation and calculate checksum of specific Packet from PCAP file. I send to this function all 20 bytes of the IPv4 layer and the result was 10 and different from the PCAP file (e2 5c) Maybe this calculation is wrong ?