This is probably due to the storage algorithm used. Read about it in the GeigerLog manual in chapter "Appendix E – GMC Device: Internal Memory, Storage Format and Parsing Strategy"
After downloading the counter memory into GeigerLog, you can call menu: History --> GMC Series --> Show History Binary Data as AA / FF Map. This will give an output like:
==== Show History Binary Data as Map of 0xAA and 0xFF ================================
from: geigerlog/data/test6.hisdb
One single printed character maps a chunk of 16 bytes of data
'A' marks occurence of value 0xAA in chunk (AA => DateTime String)
'F' marks occurence of value 0xFF in chunk (FF => empty value)
Byte No|    127|    255|    383|    511|    639|    767|    895|   1023|
      0|.........A...........A...........A...........A...........A......
   1024|.....A...........A...........A...........A...........A..........
   2048|.A...........A...........A...........A...........A...........A..
   3072|.........A...........A...........A...........A...........A......
   4096|.....A...........A...........A...........A...........A..........
   5120|.A...........A...........A...........A...........A...........A..
   6144|.........A...........A...........A...........A...........A......
   7168|.....A...........A...........A...........A...........A..........
lines removed
  48128|.A...........A...........A...........A...........A...........A..
  49152|.........A...........A...........A...........A...........A......
  50176|.....A...........A...........A...........A...........A..........
  51200|.A...........A...........A...........A...........A...........A..
  52224|.........A...........A...........A...........A...........A......
  53248|.....A...........A...........A...........A...........A..........
  54272|.A...........A...........A...........A...........A...........A..
  55296|.........A...........A...........A...........A...........A......
  56320|.....A...........A...........A...........A...........A..........
  57344|.A...........A...........A.....AA...........A.....AAA.FFFFFFFFFF
  58368|FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
  59392|FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
  60416|FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
  61440|.A...........A...........A...........A...........A...........A..
  62464|.........A...........A...........A...........A...........A......
  63488|.....A...........A...........A...........A...........A..........
  64512|.A...........A...........A...........A...........A...........A..
Byte No|    127|    255|    383|    511|    639|    767|    895|   1023|
Note the FF section in the middle separating the new data above (in the pic) from the old data below.