[exim] Strange behaviour in rate limiting

Hello,

I have a setup where we limit the sending rate for users respective IPs
to 1500 per hour. That worked fine and reliable.

However, I encountered a strange problem. Two times now I had the case
that an IP that was not send mails before start with 1500 (exact
1500.6/1h), went up to 1500.7/1h and then fast down when the IP did sent
the mail again. I have no idea where the start value of 1500 comes from.

The relevant part in the ACL config:
  deny
    !hosts = /etc/exim4/legitimed_sender
    !authenticated = *
    ratelimit = RATE_LIMIT / 1h / per_rcpt / leaky / $sender_host_address
    !domains = our.domain : *.our.domain
    log_message = Rejected $sender_host_address due over rate with $sender_rate/$sender_rate_period
    message = Rate limited

I use exim version 4.71 from ubuntu.

Anybody a idea where this strange behaviour could come from? Ah, yes,
RATE_LIMIT is set to 1500 in main section.

Regards
   Klaus
- -- 
Klaus Ethgen                              http://www.ethgen.ch/
pub  4096R/4E20AF1C 2011-05-16   Klaus Ethgen <Klaus@???>
Fingerprint: 85D4 CA42 952C 949B 1753  62B3 79D0 B06F 4E20 AF1C