Diving data loop closures altering depths

[Footleg]

Trying to track down why some underwater data was several metres higher
than a sump water surface, I discovered that closing loops between stations
at the same depth in diving data alters the depth of surrounding data. This
was not expected!

I've attached a fragment of the data which illustrates this. Without the
loop closed:

Station entrance.030 is at depth -5.30

Closing the loop:
 *equate Entrance.010 EntranceLoopNorth.019

where both these stations are at the same depth, but horizontally displaced
(a common scenario as depths are absolute but length and compass errors
accumulate in diving data), now:

Station entrance.030 is at depth -5.48

Is this a bug, or have I not understood something here?

Footleg