I don't think your 'experiment' was very rigorous,
JKKne 
Yeah, granted there are some good deals on that route but frequently not as good as the 'bait' numbers and even some cheaper than the one you found, but just to show how unpredictable it is, let's take your route again and look at those "from £4.00GBP" fares you quoted:
FR173 OUTBOUND
Free Online Check in
Depart:
Newcastle 07:55
Arrive:
Dublin 09:00
1 x Adult 4.00 GBP
Fare: 4.00 GBP
Online Check-In: 0.00 GBP
Taxes / Fees: 0.00 GBP
Total Price: 4.00 GBP
FR172 INBOUND (next day)
Free Online Check in
Depart:
Dublin 12:10
Arrive:
Newcastle 13:15
1 x Adult 4.00 GBP
Fare: 4.00 GBP
Online Check-In: 0.00 GBP
Taxes / Fees: 15.54 GBP
Total Price: 19.54 GBP
Plus Payment handling fee of £5 each way if no Electron.
Did you check the taxes coming back in your example?? (Tip you can't travel back on the same day if you haven't arrived yet, and yes the tax does vary between NIL and £15.54 on different flights even some with same bait prices!) Confused yet? Do you feel lucky? Confused enough just to switch and forget the bait?
