How unusual is this, on my tide prediction for Poole harbour last Sunday just gone, first low was 1.6m above at 11,58 and 1st high water1.5m above at 01,16.
you have approx 11hrs' differents between your two predictions,
so I believe these are 2 lo tides in stead of 1hi and 1 lo,
have a new look at your tide table,
you may find a hi tide between these two predictions
Poole is an odd tide in the harbour as there are always two highs and two lows due to the tidal effects of the harbour. The middle high tide is classed as a high even though its common at Neaps for the height of tide to be less than the preceding or following low. The main highs last weekend were at 07:04 and 18:40, but there were two lows at 11:58 and 13:16 with a little hump of a high between them that was barely distinguishable from either low.