Why should one syllable make that much difference? Let's look at the contrast between the 7 and 8 syllable line in Spanish.

The seven syllable line has this pattern:

1 2 3 4 (5) 6 (7)

There will always be an accent on syllable six, never on 7 and very rarely on 5. That gives us four syllables where the poet will place an accent. The most predictable place will be on 2 or 4, which will maintain an iambic, binary flow. 3 will give a ternary rhythm:

Let's see what happens in this poem by Brines

Vente, luz, a mis ojos, [1, 3, 6]
descansa tu fatiga [2, 6 ]
en ellos, tan cansados, [2, 6]
alíviame y acábete [2, 6]
en el amor del hombre. [4, 6]
Antes que se dilate [1, 6]
la sombra de la noche [2, 6]
en que has de morir [2, 6]
y yo morirme, [2, 5] ! [this line is a syllable short of the six-pack]
álzame tu pañuelo [1, 6]
que, tras de las montañas [2, 6]
es un fuego de rosas, [3, 6]
y dime que la vida [2, 6]
fue un día fiel, y largo [1, 2, 4, 6]
que supo de mi amor, [2, 6]
y amaré este cansancio [3, 6]

Very few lines have more than 2 stressed syllables.

The 8 syllables line has a totally different dynamic, because it will be much more likely to vacillate between binary and ternary patterns, and to have 3 rather than 2 accents per line.

Las piquetas de los gallos [3, 7 ]
cavan buscando la aurora [1,4, 7]
cuando por el monte oscuro [1, 5, 7]
baja Soledad Montoya. [1, 5, 7]
Cobre amarillo, su carne [1, 4, 7]
huele a caballo y a sombra. [1,4, 7]
Yunques ahumados, sus pechos [1, 4, 7]
gimen canciones redondas. [1, 4,7]
¿Soledad, por quién preguntas? [3, 5, 7]

The binary pattern is 3, 7, (or 1,3,7, etc...) with accents on odd-numbered syllables.

The ternary pattern is 1,4,7.

"Mixed" pattern, as defined by Tomás Navarro Tomás, are 2,4,7; 2, 5, 7. 1,5,7 is also possible.

The poet has one extra syllable to play with (1-5) and this extra room allows for a lot more variation of rhythm. There are two distinctively different rhythms, plus a few intermediate variations.

xx/x xx/x (binary, trochaic) [de cristales y laureles]

/xx /xx /x (ternary, dactylic) [Voces de muerte sonaron]

x /x /xx /x (mixed) [aguarda grietas del alba]

x /xx /x /x (mixed) [caballo de larga cola]