why mysql binlog creating new log file daily

Question

I have started binary logging on MySQL server on my Ubuntu. It start creating log files in /var/log/mysql. My my.cnf configuration is as follows:

log-bin="/var/log/mysql/"
expire_logs_days=10 
max_binlog_size=50M 
binlog-ignore-db="phpmyadmin"

Problem:

When I check logs folder a new file appears daily. Which is created at around 6:45AM to 7:00AM IST i.e 1:30AM to 1:45 GMT. But According to official documentation of MySQL on Binary Logging says:

The server creates a new file in the series each time it starts or flushes the logs. The server also creates a new binary log file automatically after the current log's size reaches max_binlog_size.

Why new log file is created daily as:

1. My system is not rebooted from last 10days which I checked with uptime command in terminal.
2. The max_binlog_size is given 50M in configuration and my logs reached only 2-3M.
3. I have not used FLUSH logs.
4. I have no CRON which do some task auto magically.

I have googled this query but didn't find the reason.

Can anyone help me to know the reason. Thanks in advance