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This is an easy one...always look for the personal motivation!
The EA wants your sales instruction.
The valuer doesn't want to get sued by the instructing bank / lender.
Ergo, one will usually 'value' high and the other...realistic (best case) or low (fear-based worst case).
EAs DO NOT value properties at all actually...they 'appraise the likely value' and this often translates as the highest possible listing price that 1) they can attract viewings and then accept offers on below to still make a sale and / or 2) woos you into thinking you will make loads of money from the sale, only to chip you down over the next couple of months, whilst hoping to retain your business.
In reality, the only valuation that matters in the end is what someone actually pays for a property...but if they are using finance (or as with you refinancing), then they will need to also persuade the pessimistic valuer to agree with them
The second best way to value a property is to assess it based on recent, relevant comparable sales values of like-for-like properties within close proximity...it's often in between the two extremes above...