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.