RollingRandomAccessFile rolling files incorrectly

Implemented Loggers using RollingRandomAccessFile appender with
this configuration:
<RollingRandomAccessFile name="SystemAppender"
filename="${fileLogDir}/system.log"
filePattern="${fileLogDir}/$${date:yyyy-MM}/system-%d

-%i.log"
append="false">
....
<Policies>
<OnStartupTriggeringPolicy/>
<SizeBasedTriggeringPolicy size="${fileSize}"/>
<!-TimeBasedTriggeringPolicy/->
</Policies>
<DefaultRolloverStrategy max="${numGenerations}"/>
...
Ran a test writing 5'000 log events (each approx. 500 bytes long) to 1 file with fileSize=100 KB & numGenerations=5.
Result: system.log file partly contained content from system-2015-07-27-5.log file (found entries 4795-5000 as expected, but unexpectedly also found entries 4783-4794 of gen. 5 file there).