Tannerman evolved over time. He was blurry, then there were details added, then he was Robert Murat, then he wasn't.
With her friends under suspicion from Amaral and the gang, Jane's imagination might've started to take over a little bit - perhaps subconsciously. It's not uncommon though. Witnesses to crimes and events often do this.
EDIT: mrjitty's offered some good commentary above on how it may have happened
here's her description from day 1:
( * ) Dark skinned individual, male sex, aged between 35-40, slim physical appearance, about 1.70m tall. Very dark, thick hair, longer at the back (she could only see him from behind). He was wearing linen type cloth trousers, beige to golden in colour, a "duffy" sic type jacket (but not that thick). His shoes were dark in colour, classic type. He had a hurried walk. He was carrying a child, who was lying on both his arms, in front of his chest. By the way he was dressed, he gave her the impression that he was not a tourist, because he was very "warmly dressed".
(**) About the child whom appeared to be sleeping, she only saw her legs. The child appeared to be older than a baby. She was barefoot and was wearing what appeared to be cotton pyjamas of a light colour (possibly white or light pink). She is not certain, but has the impression a design on the pyjamas, possibly a floral pattern, but she is not certain.
P.J. POLICE FILES: JANE TANNER'S STATEMENT 04 MAY 2008
1.70m is approx 5'7"-5'8"
Tanner said she saw the individual from approx 5 metres away - the length of a decent sized lounge room.
the guy SY said she saw is approx 6'5"
her own partner is approx 6'5"
yet she did a positive id on Murat, who was nowhere near 6'5".
there was/still is certainly a lot of unexplainable blurriness.
and then people blame the portuguese LE for a bungled investigation. it would be laughable if it wasn't so serious. moo.
edit: wrong link:! corrected