Difference between strongly universal and $\delta$ univeral hash functions Define a family of strongly universal hash functions as:

$$\forall x_1, x_2 \in \{0,1\}^n, \forall y_1, y_2 \in \{0,1\}^m, ~ x_1\ne x_2: ~~ \Pr_{h\in H} [h(x_1) = y_1 ~\text{and}~ h(x_2) = y_2] \le \frac{1}{2^{2m}}$$

if $h: \{0,1\}^n \rightarrow \{0,1\}^m$
and a family of $\delta$ universal hash functions:
$\forall x_1, x_2 \in \{0,1\}^n$ where $x_1 \neq x_2$:
$$Pr_{h \in H} [h(x_1) = h(x_2)] \leq \delta$$

(Definitions according to Moni Naor slides.

Then I see why strongly universal implies $2^{-k}$ universal (simply pick $y_2 = y_1 = h(x_1)$), but according to Moni Naor slides, $\delta$ universal does not imply strongly-universal.

Since I don't fully understand the counterexample on the slides ($h(x) = x$) I am searching for a counterexample and an intuitive description of the differences between the two definitions?
There are two orthogonal question axes here:


Universal vs. strongly universal.
A universal hash family has bounded collision probability: for any inputs $x \ne y$, the probability that they collide under a random hash function $H$ is bounded by $1/t$, where $t$ is the number of possible hash values: $$\Pr[H(x) = H(y)] \leq 1/t.$$  (If hash values are $m$-bit strings, then $t = 2^m$.)  However, although the probability of collision may be bounded by $1/t$, knowledge of $H(x)$ may inform you about $H(y)$ for certain pairs of $x$ and $y$.

In a strongly universal hash family, sometimes called pairwise independent, this does not happen because the hash values of any two inputs $x \ne y$ are independent uniform random variables: $$\Pr[H(x) = u, H(y) = v] = \Pr[H(x) = u] \cdot \Pr[H(y) = v] = 1/t^2,$$ for any hash values $u$ and $v$.  This is called pairwise independent because it may be limited to any two variables—for any positive integer $k$, there's a corresponding notion of $k$-wise independence.

Obviously any pairwise-independent hash family is universal (proof: exercise), but the converse does not hold.  For example, fix a prime $p$, and define $H_1(x) = a x$ on $\mathbb Z/p\mathbb Z$ for uniform random $a \in \mathbb Z/p\mathbb Z$.  Then $H_1(x) = H_1(y)$ means $ax = ay$, an event which, for $x \ne y$, happens if and only if $a = 0$, so $$\Pr[H_1(x) = H_1(y)] = \Pr[a = 0] = 1/p.$$  But if $H_1(x) = a x = u$, there is only one possible value of $H_1(y) = a y = v$, namely $v = y u/x$, so

\begin{equation*}
     \Pr[H_1(x) = u, H_1(y) = v]
     = \begin{cases}
         1, &amp; \text{if $x v = y u$;} \\
         0, &amp; \text{otherwise.}
       \end{cases}
   \end{equation*}

Hence $H_1$ is not pairwise independent, because for certain values of $x$, $y$, $u$, and $v$, $\Pr[H_1(x) = u, H_1(y) = v] = 1$ is far above the bound of $1/p^2$.  That is, if you know $x$ and $y$, and you learn $u = H_1(x)$, you can perfectly predict what $v$ will be even if you didn't know a priori what the secret hash key $a$ was.
Universal vs. $\delta$-universal.
This is just a generalization of the concept: you replace the bound $1/t$, for a hash family taking on $t$ possible hash values, by the bound $\delta$, which is usually a small multiple of $1/t$.  There is a corresponding notion of a $\delta^2$-strongly-universal hash family.  A $1/t$-universal hash family is just universal.  A $1/t^2$-strongly-universal hash family is just strongly universal.

For example, fix a prime $p$, say $2^{130} - 5$, and let $r \in \mathbb Z/p\mathbb Z$ be uniform random.  For a polynomial $f$ over $\mathbb Z/p\mathbb Z$ of degree at most $\ell$, define $H_2(f) = f(r)$.  We can encode a message as the coefficients of the polynomial $f$.  If $H_2(f) = H_2(g)$ for polynomials $f \ne g$, then clearly $f(r) = g(r)$, so $r$ is a root of the polynomial $f - g$.  But there are only at most $\ell$ such roots.  Since every $r$ has probability $1/p$, we have

\begin{equation*}
     \Pr[H_2(f) = H_2(g)]
     = \Pr[r \mathrel{\text{is a root of}} f - g]
     \leq \ell/p.
   \end{equation*}

Thus, $H_2$ is $\ell/p$-universal.  Here $\delta = \ell/p$ is a small multiple of the number $1/p$ of distinct outputs from $H_2$.  This hash family $H_2$ is noteworthy in cryptography because it is the basis of the Poly1305 message authentication code, which is a contender (along with GHASH) for the most popular MAC in the world.  (Some background on the history and role of universal hashing in message authentication codes in cryptography.)