RTP timestamp in data packets vs RTCP SR packets

I'm trying to understand the difference between the RTP timestamp as it occurs in RTP data packets vs as it is used in RTCP Sender Report (SR) packets.

For the RTP timestamp in data packets I have established that:

• They are not based on wall-clock time but represent more of a counter
• They typically have a random offset chosen at the beginning of a session
• For simple standard audio codecs they typically increment by 160 (roughly speaking 1000ms / 20ms * 160samples = 8000 bit rate) and this increment also includes not sent silent packets

For the RTP timestamp in RTCP sender report packets I originally thought they were just a snapshot of the current RTP timestamp of the data packets and in conjunction with the NTP timestamp (which is typically wall-clock) could be used to calculate the wall-clock time of further incoming RTP packets which is also what I understood from this analysis on the subject.

However a sentence of the RFC 3550 Section 6.4.1 makes me wonder about that assumption:

Note that in most cases this timestamp will not be equal to the RTP timestamp in any adjacent data packet.

This ruins my assumption, because I assumed that the SR packet would contain an RTP timestamp that is found in a data packet that has just been sent by the same source. Unfortunately the next sentences are pretty much meaningless for me (maybe this is an English language barrier but it sound like non-helpful nonsense to me)

Rather, it MUST be calculated from the corresponding NTP timestamp using the relationship between the RTP timestamp counter and real time as maintained by periodically checking the wallclock time at a sampling instant.

Could you clarify for me how the RTP timestamp in an RTCP SR packet can be calculated?

The process of sending RTCP report packets is separated from sending the related RTP packet stream(s). With that I mean that they usually won't be sent at the same time. Therefore a sent RTP packet and RTCP report packet will typically contain different RTP timestamp values.

As you know a relationship exists between the NTP (wallclock) time and the RTP timestamp. Since RTCP report packets contain both an NTP timestamp and an RTP timestamp these packets can be used to learn how these values relate at the side of the sender of the RTCP packet. Any RTP packets received from the same sender will contain their own (typically different) RTP timestamp. Using the relationship learned from the received RTCP packets this RTP timestamp can be used to calculate the wallclock time of the moment the RTP packet was sent.

The answer to this stackoverflow question might also help you.