EDIT: You said that the full memory buffer is used to take measurements, it means that the scope measures the period of each cycle and then displays the mean of all these periods?
No.
Take example. There is 20ns cycle time (50MHz)
1ms/div, 14M acquisition length. 1GSa/s sampling.
Automatic measurement can show 50.00MHz and 20.00ns cycle time. It is mesured over one cycle. It do not count every 700000 cycle what is in buffer and then average these. Scope need keep speed as fast as can. in perfect system there need regognize and measure every pulse, every zero crossing and it may need interpolation if we some day want measure pulse width also because every single pulse there can be different.
This is only rare case that we are looking low jitter continuously repeating uniform signal. What it show if there is bursts or other freq changes. But what we here know, even if we can not see signal (my first image) There is trigger and depending settings and signal I have trigged to some place. If signal is not repetitive mostly still im most interest to point where I have trigged. It mesure this cycle there afaik.
For avoid big confusiuon in many different situations I do not recommend averaging over memory length. (exept if there is special new function and settings for this kind of frequency measurement special function). But soon we ask then if we can get also input for external frequency reference.
But it also full memory and resolution mean that even when 1 div on the display is 1ms it have 1M sample points (but only 50 display points)
(1000000ns) so it have still 1ns resolution available for measurements. Many scopes measure only from display memory or some second buffer where is decimated sample points, example in Siglent 1000X models it looks like this second buffer is 70k perhaps) and if look Rigol DS1000Z it may have something like 300- <1k) points resolution.
But then, we have 14Mpoints buffer now and 1ms/div in use. With this 50MHz it use only 20ns slice--- for detect this freq and width (near trigger) Thisd is big advantage over many these scopes what use second buffer or even only display memory resolution.
But there is also cases where also this full length is important. Lets keep 1ms/div.
Set pulse length 13.99ms. And it show +width is 13.99ms. (trigger just near left border) It show rise and fall times right 6ns
And here come now RANTING. I want now these nanoseconds also for pulse length because they are there and you advertise full resolution! Why Siglent reduce this result far below true resolution what is now there. Ok I know there is more decimals and room in display is small. This display area can really use much more clever way) It show this 13.99ms pulse (+width) and that it have 6ns rise and fall time. Why it do not show 13.990005 ms or 13990005ns because it was this and also there is this resolution available in memory. (this is not only place where they trunkate numbers without real reason, but please do not this 0.000000ps stubidity, it is reserved only for Rigol)