Vanitygen: Why is it easier to generate an address that starts with a capital letter? I get a much lower length of time if i make the first character a capital letter instead of a lowercase letter, when trying to generate a public key with Vanitygen.

Shouldn't it be the same? Has anyone else experienced this? What is the math / logic behind this difference?
I'm copying this from deepceleron's post on bitcointalk:


  The change happens at a particular address:
  
  prefix difficulty: 77178 1QLa 
  prefix difficulty: 78362 1QLb 
  prefix difficulty: 4553521 1QLc 
  
  This is a quirk of how the 25-byte (50 digit hexadecimal) Bitcoin
  address is converted into Base58 (represented by numbers and letters),
  and the different maximum values that can be stored in 25 base256
  digits vs 34 base58 digits.
  
  The Bitcoin address in it's native binary form (that you never see) is
  25 bytes, it's parts are:
  byte 0: Network ID Byte (0x00 for main
  bitcoin network)
  byte 1-20: ripemd160 hash (20 bytes) of sha256 hash
  (32 bytes) of 0x04+public key (65 bytes)
  byte 21-24: checksum: first
  four bytes of sha256 hash of sha256 hash of bytes 0-20 above
  
  This would be 50 hexadecimal characters long (base16), with a possible
  digit value between 0-F.
  
  We will ignore byte 0, it is always 0x00, and Base58 conversion always
  preserves this leading 0 byte by directly encoding it as a "1".
  
  That means the "vanity" part of the address is bytes 1-24, 24 bytes of
  random hash output, or 48 hexadecimal digits. The maximum value that
  this can be is 0x FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF
  ffff ffff (the lower case "f"s are the checksum, and won't be all
  FFFFs for the maximum ripemd160 value.) We can guess from the output
  below that this maximum possible address value encoded in Base58
  becomes 1QLbxxxxxx.... something:

1QLa8LNNFeYs7KJWvxMdR5YtRjkwtQtkyb
1QLabhman6ZQyz3musbqV1RjFmE3pyT29j
1QLaJHv2LCSYo8WgT9qPf4rdttduRGyozX
1QLarwzJCUWGuQEZ4uWPsYVz8N2P5XmeQU

1QLckWG2tx17suupYU2FfeWunTCZMuJKW
1QLcMk7NVLwvg8gw2ihJTuG8x7WLYCjMg
1QLcLr6AoN687CpH1JZnfeMmWvUka5NHX
1QLc47945bSCv4J1Z4yRuvUbtooQqmGJ1
1QLcU9NNxaqfvqMRXD8dFtL2SktL3YmBT
1QLc1D24uDbvy327MzifEChVvATAeDsbF

  
  See how the 1QLa addresses are full length, but the 1QLc addresses are
  one digit shorter? That is because the only way to have an address
  starting at 1QLc or greater is by having a binary address that is 59x
  smaller.